Claire Eastham is 26 years old, and truly blessed: funny, lovely to look at, extremely bright (she has two degrees). She lives with her boyfriend in London and has a job she loves ("my dream job") at the publisher Penguin where she works in sales. Appearances, though, can be deceptive. Last January, she had to take a month-long leave of absence from work. The panic attacks from which she has suffered since she was a teenager had started to dominate her life. "I thought: I need to do something about this, because panic attacks are the worst. You feel like you're going mad, like you're going to die; worrying about everything, feeling out of control, wondering what you sound like and what you look like. The voice in your head, it's constant. You can't stop it. It's exhausting."

Determined to try to get better, she signed off sick – "I was devastated to have to do it because I thought: the game's up, everyone will know" – and went to see her GP. What did he say? She smiles. "He told me to drink camomile tea." Anything else? "Yes, he prescribed Valium and Sertraline, a drug I've taken before [a mild anti-depressant, Sertraline is also known to be effective in treating social, obsessive compulsive and panic disorders]. It does calm you down a bit, but I didn't like the idea of having to rely on pills for the rest of my life, and that's when I realised: I'm going to have to get myself out of this. The pattern with my anxiety has always been that eventually it goes, only to come back later even harder." Somehow, she had to face it down, and send it packing.

In the end, it was a charity, Anxiety UK, that came to her rescue. "I contacted them, and it was amazing to find that I wasn't alone, that in itself helped a bit. But they also gave me good advice. They recommended I start using exercise to take the edge off the adrenalin, and they suggested Cognitive Behavioural Therapy (CBT). I had 10 sessions, which was all I could afford, and it changed everything. It wasn't only that I was able to be honest about how I was feeling, that off-loading on to someone who understands, as opposed to someone who just thinks 'Oh, snap out of it', felt so good. The therapy gave me the tools I need to rationalise. You have to commit to CBT; you have to put the time aside. But it does work. I still have bad days, but at least now I have a strategy."

The CBT taught her to challenge the voices in her head. "The thoughts I have… there's a formula: you notice the same ones again and again. For instance, at work, the voice says I'm a loser, weird, bad at my job, that it was only thanks to luck that I got it at all. Someone would speak to me, and instead of listening to what they were saying, all I could think was: I need to get out of here or I'll make a fool of myself."

Her therapist taught her to write down these thoughts and then to identify her thinking errors – for instance, the way she is prone to catastrophise, always jumping in her mind straight to the worst possible scenario. "You write down your emotions, and you ask yourself questions. Would a scientist accept this way of thinking? Would a judge? Where's the evidence? Then you write another list, a rational one." She points to her iPhone, on the table between us in the bar where we meet. "I use an app for this now: Thought Diary Pro. It's great. It's always in your bag; you don't have the embarrassment of getting out a pen and paper. I've also got distraction techniques that I use, especially at night. I'll go through all the names of the characters in Sex and the City. It gets me off the track of circular thoughts. Or if my boyfriend is around, and I'm having what I call a wobbler, he'll suggest a game."

‘The voice in your head is constant. You can’t stop it. It’s exhausting’: blogger Claire Eastham Photograph: Suki Dhanda for the Observer

Where does Claire believe her anxiety comes from? She isn't sure – though, like Freud, she defines her anxiety as a threat that is objectless, and located in the future – such as ruination or humiliation (unlike fear, which is a response to a specific and immediate threat to one's safety). There is no history of panic attacks in her family, and she had a happy childhood (she grew up in Bolton).

"It started when I was 15," she says. "When I realised that I wasn't pretty or intelligent, or as good as I thought I was." For a moment I think she's joking, but then I realise, no, she really isn't. "It was all about comparing myself with other people, I suppose. The first thing I remember was that all of a sudden, if anyone spoke to me, I would start blushing really badly. Then I started getting tremors, and that was when I went to the doctor. I thought: I can't do this any more. I wondered if it was a hormonal thing. The doctor told me straight away that it was anxiety, but that there was nothing to be done about it, and that I would grow out of it.

"When I left school, it went away for a bit. I don't know why. But in my first year at university, it started getting worse. That was when the overthinking started, the racing thoughts. Bad insomnia, lots of headaches, nausea. I would physically brace myself, hold my breath until my stomach began to hurt. I thought it was depression. I was really emotional and unhappy. Again, I went to the doctor. He prescribed Sertraline, and then I just got on with it. I thought: This is who I am. When I got my job in publishing, I really wanted to do it. But it was a massive change. I had to move to London where I didn't know anyone. I was able to work, but I used to take weeks off, though I never said why. I think now that it's change that triggers it. Any new environment reminds me of school, and I associate that with teachers who used to call me thick and bone idle, because I used to daydream, because I wasn't quite there. Even now, it doesn't matter that I've got two degrees – I still think I'm stupid. I constantly have to remind myself that I'm not."

It has been great, she says, coming clean to her employer – everyone has been so supportive. She feels liberated. Hiding her "craziness" was becoming more exhausting than the anxiety itself. So can she envisage a time when her anxiety will belong only to the past? Not exactly. "I'm encouraged by how much better I've got. I can't believe what I used to put up with. But it will always be there somewhere. What if it comes back? That's my worry now. I call it the tiger. It pounces on you when you least expect it, and it's so hard to shake off." These days, she tells me, she takes good care of herself. Allowing herself to get run down is not a good idea. If she has a cold, or a hangover, she can feel her anxiety lurking. It waits, looking out for an opening, for some small chink in the defences she has built up so very carefully.

Anxiety is not A DISEASE of the 21st century. As the American journalist Daniel Smith points out in Monkey Mind, an anxiety memoir that went on to become a New York Times bestseller, Freud wrote a book about it 90 years ago (The Problem of Anxiety), and Kierkegaard 80 years before him (The Concept of Anxiety), and Spinoza was the father of them both (it was in the 17th century that the Dutch philosopher noted our enslavement to what he called "dread"). Victorian novels are replete with characters – particularly women characters – who exhibit what we might recognise now as some of the symptoms of anxiety disorders, from fainting to hysteria: manifestations of inner turmoil that would, in real life, have had the phrenologists running to examine their heads, and the hydropathists rushing to welcome them to their new-fangled spas (cold-water remedies were particularly popular when it came to treating what our ancestors regarded as a form of madness).

Franz Kafka, a writer whose name has become synonymous with a certain kind of alienation, described his own paralysing anxiety as "the feeling of having in the middle of my body a ball of wool that quickly winds itself up, its innumerable threads pulling from the surface of my body to itself" – an analogy that has, I think, yet to be improved upon (as I found while I was thinking about this piece, dread is difficult to articulate, and even harder to pin to paper).

‘I call it the tiger. It pounces on you and it’s so hard to shake off’: Claire Eastham. Photograph: Suki Dhanda for the Observer

But still, somehow it feels new: late 20th-century psychiatry has officially recognised chronic anxiety as a serious illness, and divided it into subsets (the term covers a whole range of disorders from obsessive compulsive disorder to panic disorder to post-traumatic stress disorder). Thanks to improving diagnosis and official statistics, moreover, we can see just how common it is. Last month, a survey by the Office of National Statistics revealed that levels of anxiety in Britain had dropped by almost 1%. However, this was hardly good news: 20.9% of people still rate their anxiety levels at 6 or more out of 10, while the general consensus among psychiatrists is that between 10% and 30% of the population is likely to suffer from an anxiety disorder at any one time.

Mental illnesses such as anxiety and depression – there is comorbidity between the two – cost the UK economy some £80m annually. Anxiety disorders are painfully debilitating, their symptoms and the rituals involved in managing them causing sufferers a good deal of shame and embarrassment. Those who fail to seek treatment – and many still do – are at a higher risk of committing suicide. Naturally, there are those who like to talk about the "medicalisation" of a perfectly ordinary human emotion, one that we need in order to survive. But once you begin talking to sufferers and to experts, it becomes pretty clear that these people have very little idea of what they're talking about. As one psychiatrist put it to me: "Those who endure anxiety aren't putting this on. They're not being dishonest. Discovering that what you thought was a heart attack was in fact a panic attack doesn't invalidate your suffering. I am absolutely certain my patients are in great pain."

Has the recession made things worse? Yes – though there is only anecdotal evidence for this (the Adult Psychiatric Morbidity Survey, which compiles the statistics used by charities such as Mind, is only published every five years, and the next one is not due until 2014). "Since the downturn, we've seen increased calls to our helpline and increased requests from people wanting to access support services," says Nicky Lidbetter, the chief executive of Anxiety UK. "What we are finding is that people who might ordinarily have managed their anxiety quite well have been tipped into new territory by being made redundant or having to adapt to new life circumstances." Are those who are generally quite anxious more likely to develop anxiety disorders? "There isn't an answer to that."

So why do SOME people cope with worry and stress, and even thrive on it, while others find that it makes them ill? The nature/nurture debate rages on. Dr Christos Dimitriou, a consultant psychiatrist at the East London Foundation Trust, who is broadly on the side of nature, likes to use the analogy of a suspension bridge. "Each support strut represents a gene, and the bridge represents your entire genetic loading. Everyday stressors are represented by cars, and major stressors –divorce, bereavement, redundancy – by lorries. In the absence of any faulty genes, the bridge can handle all the traffic, but if there are a few faulty genes, the respective support struts will be weakened, and less able to cope with it, particularly the lorries."

Others dispute the role of our genes. Oliver James, the writer and clinical psychologist, believes that it's those who had a difficult childhood who are most vulnerable. They grew up in a state of permanent "red alert" which deregulated the cortisones in their brain chemistry. Combine this with what he calls our "toxic society", and the struggles of life are hard indeed to weather.

But whatever the cause, there can be no doubting the effects of such disorders on a life. Yasmin Jade, 21, who has suffered from panic disorder since she was 13, currently finds herself housebound in Birmingham, her symptoms – shortness of breath, trembling, blurred vision, nausea – having recently overwhelmed her (she has dropped out of university).

"I don't enjoy sitting at home all day long," she says. "But I physically can't do anything else at the moment. It's as though a pause button has been pressed on my life. I'm just... waiting." She finds herself in a downward spiral – the anxiety makes her feel low, and the lower she feels, the more vulnerable she is to her anxiety. (It's this spiral that leads many GPs to misdiagnose anxiety – they mistake it for depression.)

‘Since the downturn, we’ve seen increased calls for help’: Anxiety UK’s Nicky Lidbetter. Photograph: Jason Lock/Anxiety UK

Nicky Lidbetter now runs two mental-health charities, yet still her life is impaired by the agoraphobia and panic attacks she first suffered as a student. "I've achieved within the constraints the disorder has placed on me," she says. "But I continue to find it hard to travel. I live my life within a 50-mile radius. Anxiety UK is based in Manchester, but my parents live in Peterborough. The other day, I put my children – they're 10 and 15 – on a train to visit their grandparents. It was horrendous. I wasn't even getting on it, but I still had a strong emotional reaction. If I wanted to go and get them I wouldn't be able to, and that's awful really.

"My daughter is going to university soon, and her attitude is, I'm not choosing where I go on the basis of whether you can travel there. So we did an experiment. We caught a train from Manchester to Stockport. It was the first time I'd been on one in 23 years." How did the short journey make her feel? "I didn't like it. I thought: how long can I contain this anxiety before it completely overwhelms me?"

The vast majority of sufferers are in the same camp as Lidbetter – able to function, but in relatively painful fashion, their lives ruled by avoidance, subterfuge, ritual (for instance locking and unlocking the door several times) and self-medication (they may drink, or self harm). Daniel Smith refers to these sufferers as "stiflers". "They learn to seal their anxiety off from public view," he writes in Monkey Mind. "They learn to cork their anxiety within themselves like acid in a vial. It isn't pleasant. The human mind isn't Pyrex, it can corrode. But it works." In his book, Smith devotes several hilarious but ultimately deeply sad pages to his struggle to hide his extreme perspiration from his colleagues – a battle that culminated in him electing to wear sanitary towels under his arms before any important meeting.

Smith's illness was, at times, completely debilitating. He had to leave a job he loved, and he and his girlfriend separated. Even he, though, found that therapy combined with changes to his work life improved his mental health in the end. And this is what all the experts tell you – anxiety might be a chronic illness, with an extremely long life span, but it is also eminently treatable.

"I'm not a deluded optimist," says Dr Dimitriou. "People can get over this, and they should seek help before the disorder reaches the stage where it incapacitates them." He believes the best results are achieved by combining medication and psychological interventions. "Talking therapies really can influence the robustness of the particular neural pathways that counteract anxiety. In other words, they can alter anxiety at its root. The therapies that we use are CBT based, but CBT has developed a lot. There are variants to suit particular presentations, and they're really very sophisticated." Nicky Lidbetter, however, is more cautious. "CBT is very much flavour of the month. It's Nice-approved. But you have to bear in mind that the NHS provides only six sessions per patient, and waiting times do vary. Also, that it doesn't suit everyone. It has an 80% success rate. What about the 20% it can't help?"

Some patients, she believes, respond better to graded exposure therapy, in other words, by trying, bit by bit and in a managed way, to do the things that scare them most (40 years ago, anxiety and phobias were treated by means of "flooding", ie prolonged exposure to anxiety-inducing situations. Graded exposure therapy is a less brutal form of flooding). Anxiety UK also recommends exercise – "we can't emphasise enough the importance of exercise in using up excess adrenalin" – and that sufferers avoid sugar, caffeine and alcohol, all of which can make a person more jittery.

The self-help literature available is extensive, and much of it very good. Claire Eastham recommends the Panic Attacks Workbook by David Carbonell, the founder and director of the Anxiety Treatment Center in Chicago. She also thinks that humour plays an important role in recovery. She has learned, over time, to laugh at herself. Finally, social media may be worth avoiding if you are chronically anxious. "Facebook is such a competition," says Eastham. "On there, you're your own celebrity."

Perhaps I'll add a personal note here. I can't claim ever to have suffered from a full-blown anxiety disorder. Most of the time, I'm able to manage my worries. I ask myself, what's the worst that could happen? and usually, the answer comes back to me, cool and sane – nothing too bad. When an anxiety did, for me, turn into a full blown phobia a decade ago, I found myself unable to drive on a motorway. I had only to see the blue sign for the M1 to have palpitations and sweaty palms. But I had a course of CBT, and, within weeks, I was driving north again. All I can tell you is beating my phobia felt fantastic. I was euphoric. No one should have to spend their days being afraid of being afraid.

Claire Eastham blogs at weallmadhere.wordpress.com