Single women in their 40s and 50s are increasingly feeling that their love lives are over as men their own age use online dating to cherry-pick younger models. But when did confidence and sexual maturity become so unattractive?

When a divorced woman on "the wrong side of 45 with a brace of kids" began to write about her experiences of being single last week, she opened her blog with the extraordinary statement that she was in "relationship no man's land", condemned to be alone for the rest of her life.

"I am," she wrote, "a plankton on the food chain of sexuality and the prospect of a relationship."

The anonymous woman, whose blog is called The Plankton, is not alone in believing that there are problems specific to being a single woman in middle age. A survey this month found eight out of 10 women over 50 think they have become invisible to men. Seven out of 10 women in the study felt overlooked by the fashion industry, while three-quarters of women in their 60s believed they had lost their identity by being labelled as a "mum".

Women and men are living longer and fitter lives; the average age at which we divorce is rising – 41 now for women and 43 for men – and the number of single parents is projected to rise to 1.9 million over the next decade. There is a new demographic of confident and experienced women, at their sexual peak as far as science is concerned, who would like to find a partner.

But life, friendship and love for the single woman in her mid-40s and beyond has its own particular complications and sorrows.

Susan Quilliam, a relationships expert and agony aunt, said that some women were suffering "terribly".

"On dating sites men have the pick up and down the age range. They are also much more in a rush to get into a new relationship and are much less likely to give someone a second chance, which may seem callous but they are much more likely to fall in love quickly. For men, it's a case of you fulfil the criteria, let's buy the double duvet.

"Women are more cautious. It's a shame men aim for the younger age range because women of 45 and 55 are arguably much more sexually mature and able to give a lot more pleasure than, say, a woman of 25."

The author of the Plankton blog sums up the emotional aftermath of her divorce in bleak fashion: "I may live till I am 90, but a sort of death has already come. I am already in a wilderness – maybe [facing] my time again, over 40 years, it's possible, but with no one."

She points to a passage in the book Intimacy by Hanif Kureshi, when his narrator briefly considers the fate of the woman he is leaving: "A lone middle-aged woman with kids doesn't have much cachet. She will, unfortunately, become the recipient of sympathy. At dinner parties divorced men will be placed next to her."

But according to many singles, even getting invited to the dinner party can be tricky when people tend to socialise with other couples as they get older and settled into marriages and parenthood.

Katie Sheppard, the director of relationships at Match.com, said online dating was now the second most common way couples met across the UK – behind being introduced by friends or family – and for older people it can be a perfect way to "dip a toe back into dating".

Its research shows that dating is, especially for divorced women, fraught with complication, anxiety and worry. Looking for second-time love when children are a first priority is a challenge. Nicola Lamond, Netmums spokeswoman and mother, said: "Being a single parent can be pretty tough. Single parents describe themselves as lonely, isolated, vulnerable and worthless. There is a real sense their world has shrunk."

Trying to meet people in bars and clubs can feel like a younger person's game, especially when you need a single friend and a babysitter to get there.

But nor does everyone feel that they will find love on the web.

Susan Broom is a bubbly woman with a ready laugh who is 48, single, and has now given up actively looking for a man, certainly online, and is not afraid to admit that she does feel the sadness of that.

"I gave up internet dating when I realised it was only older men who were contacting me, which doesn't really interest me. I prefer men of my own age. But a lot of those men my age are only looking at women in their 20s or 30s.

"It can be a full-time occupation when you have to really fight to meet the sort of men that are worth meeting because they are the ones refusing to look at you as a potential date. Because of its 'anything is possible', 'sweet-shop' appeal, online dating just encourages men to cherry-pick their ideal – usually younger – mate. Which is depressing if you're a woman of a certain age."

Studies across all cultures and nations have shown a consistent trend for men to have younger partners. An unhappy pattern for 45-plus women who want a new partner.

"I do feel it's not really talked about very openly," said Broom. "There are a lot of women out there who are in their late 40s and early 50s who don't have children, but would happily have had them if they'd met the right person. It's an invisible band of women, they don't complain and they just get on with their lives, but actually it's tough for them.

"I have a very antisocial job, baking cakes, and even when I'm out selling at farmers' markets I talk to lots of lovely men, with their lovely wives and children beside them. It's hard to meet people, especially in London.

"It's even harder once you're past a certain age. I'd like to try to set something up for the single people in my area, I know they are out there. A physical event where people actually meet instead of everybody finding online dating a bit of a letdown and just staying home feeling sorry for themselves."

Women also report losing friends because of the differences between single and attached lives. "As your friends settle down and have kids you can feel quite ostracised," said Broom. "Last year I had a partner for about nine months or so, and during that time I noticed how I got invited to dinner parties again.

"People really want couples round for dinner. So it's tough because you also have to get out there and find a new bunch of female mates too, and, of course, they end up being younger than you, and then you worry about going out with them and they'll be getting chatted up and you won't!

"I'm a bit burnt out with the whole dating thing right now, but I would like to see the issue being talked about a lot more. And in a real way – not just in a 'singledom rocks' way. Because quite often it really doesn't."

There are rewards, however, for remaining single, says Kate Grussing, the founder of the management consultancy firm Sapphire Partners, who believes single, childless women in their 40s and 50s have huge advantages at work.

"They're in a really strong position. They have proved themselves, they are good at what they do and at the top of their game.

"Women are more loyal to a company than men, they don't follow the biggest pay cheque and companies are starting to wake up to that and to develop and look after those kind of women. And they will have got where they are in their careers by juggling far fewer balls on the way up."

When actress Lynda Bellingham, 62, met her third husband, Michael Pattemore, in 2008 she assumed "he wouldn't fancy me". She said: "Reaching 50 really does change the way you're regarded by society. Reaching my 50s was an unsettling time, and I felt that my romantic life was over."

And that kind of assumption is something which holds a lot of women back, claims Julia Macmillan. She is on a mission to help women to tackle their 40-something love drought by doing what men do – going younger.

Macmillan, 49, has set up toyboywarehouse.com. Younger men are nowadays desperate to meet older women, she claims, and she has more men than women looking for love on her site.

"We have 26,000 members and growing, and 70% are men. Our battle is to change women's mindset and get women to believe that there are younger men who want what they are, experienced, strong-willed, confident. Men in their mid-40s slow down, women are dynamic and full of energy. We need to match that energy.

"We've found that our most successful age gap is around 10 to 15 years, although we only specify a minimum of one year. We have a lot of happy matches."