Joanna Cannon’s to-do list for next month includes attending the British Book Industry Awards in which her bestselling debut novel has been shortlisted, and pouring tea for people with dementia and their carers in a village hall.

Cannon, author of The Trouble with Goats and Sheep, who quit psychiatry more than two years ago to concentrate on writing, is returning to the NHS as a volunteer with Arts for Health. Its programme, run by South Staffordshire and Shropshire Healthcare NHS foundation trust, brings creative arts to patients. Tea-making aside, she is keen to help patients understand their own life story through “reading for wellbeing” groups and creative writing.

Cannon once retreated to read for two weeks solid after her mental health deteriorated while working as a junior doctor, and she started writing “for therapy” on a blog. “I wrote that blog to understand myself more,” she explains. “If you read some of the early entries they’re all full of doom and gloom because I was trying to process the things I was seeing as a junior doctor. Not by talking directly about them, obviously, because that would be unethical, but by talking about my reaction to things. I think reading and writing is the best way of understanding your narrative from a safe position… reading is also an escape more than anything else. When I had that two weeks off and read books I felt as though I’d left my own life for a bit and just enjoyed being somewhere else, and I think that’s important for patients. Also creative writing will give them an outlet to talk about how they feel without actually having to talk about how they feel.”

The only child of a plumber and a giftshop owner, Cannon grew up in Derbyshire and left school at 15 with one O-level. It wasn’t until her 30s that she took her A-levels, spurred by her resolve to become a doctor. She qualified in her early 40s but became very stressed during her first stint as a junior doctor in general medicine.

“I thought, ‘yes, I can do this, it’s fine, I know what to do,’” she recalls of her first job – to certify a death certificate. “But when I got there it really impacted on me being in the room with relatives and talking to them, so I just went to the loo and cried. And I thought, ‘that’s fine, this is my first time and it will get easier’. But it didn’t, it got worse. I spent my whole weekends going over and over everything that happened.”

At the time, she told no one how mentally unwell she felt. “It’s very difficult to talk to a consultant you don’t really know that well and admit that you’re struggling, because everyone else appears to be coping ... and the consultants clearly cope … so to hold your hand up and say ‘actually, I can’t deal with this’ – you feel weak and stupid and a failure that all this money has been poured into your education to get you to this point and you’ve squandered it by not coping.”

She lauds Prince William and Prince Harry for talking about mental health as part of the Heads Together campaign. “People say the stigma is lifting, but I don’t think it is. You talk to somebody with schizophrenia or bipolar or depression or anxiety and I don’t think they would think it was lifting. The NHS does try and encourage people to talk more, but you are only reflecting the general attitude that it’s just too difficult to admit to it sometimes. I’ve so much admiration for Prince Harry. It’s amazing to have such a massive public platform and get on that platform and speak about something like that.”

Cannon is all too aware that encouraging people to speak up needs to be matched by services to meet need. As the general election looms, this former NHS worker echoes the call for more funding so that mental health services don’t “bend and break”.

“The lack of funding is unbelievable. You will get people admitted as an emergency and the only bed available for them is 200 miles away. And these people are perhaps psychotic, so being taken to somewhere they don’t know, and looked after by people they don’t know, is not going to be very helpful to their recovery. In community mental health, you get community psychiatric nurses who have got hundreds of patients on their caseload; how are they supposed to spend quality time with all of them? They can’t possibly do it. And they burn themselves out and they leave. It’s tragic, really.”

Improved investment in the wider social fabric is also needed to promote good mental health, she argues. “One of the biggest risk factors for mental health is social isolation, so now where do people go for that community? Where do they go to socialise when you are cutting all these services in the community?”

For Cannon, the stress she experienced in general medicine evaporated when she joined a psychiatric team. It felt like “coming home”, she says. “I felt truly useful for the first time in my life.”

Working with mental health patients was also the inspiration for her novel, which is about prejudice towards people who are a little bit different. It took just nine months to complete – a feat accomplished by writing at 4am before her shifts began and in NHS car parks during lunch breaks – and has sold more than 100,000 paperback copies in the UK.

She secured a six-figure deal for her second novel, due out next January, and will be working on her third when not volunteering in her old professional stomping ground. “Every day in psychiatry I felt as though I’d made a difference to somebody, I’d made them feel a little bit better about life – and I miss that feeling,” she says of her decision to return to the NHS. “It’s not at all altruistic me going back on to the wards because I get as much benefit out of it as they hopefully will get out of me. I love the people, I love hearing the stories, I love the teamwork. Writing is very isolating sometimes. I miss the camaraderie of being on the wards and of being with a team.”

Has she kept the door open to being a doctor again? “I would never say never, but if I could do this,volunteer and do my writing, that would be perfect – the best of both worlds.”

Curriculum Vitae

Age: 40s.

Lives: Ashbourne, Derbyshire.

Education: Denstone College, Uttoxeter; University of Leicester Medical School (graduated 2010 with a degree in medicine).

Career: 2014-present: author, 2010-2014: NHS doctor specialising in psychiatry, South Staffordshire & Shropshire Healthcare NHS foundation trust; bar maid, kennel maid, pizza delivery expert.

Public life: Volunteer, Arts for Health (South Staffordshire & Shropshire Healthcare NHS foundation trust)

Interests: Reading, walking my dog through the fields, medical humanities, the bridge between art and science.

The Trouble with Goats and Sheep is published by Harper Collins (£7.99). To order a copy for £6.15, go to bookshop.theguardian.com or call 0330 333 6846