A fifth of cancer patients experience post-traumatic stress disorder (PTSD), a Malaysian study has found.
About one-third of these still had consistent or worsening PTSD four years after diagnosis.
The researchers said PTSD needed to be identified, monitored and treated early.
Becki McGuinness, who was diagnosed with bone cancer, said the resulting PTSD and depression were the biggest challenges for her.
Becki was treated with chemotherapy after being diagnosed at the age of 21. When it didn't work, she was told she would need radiotherapy.
"By the time I'd finished radiotherapy, and a few months passed, my periods were stopping.
"It wasn't until I was 23 that I found out that I'd gone through the menopause and was infertile," she said.
'I could have saved your fertility'
Becki was devastated - even more so when she found out it could have been prevented.
"I was waiting a whole month for my treatment. I later saw a gynaecologist who said, 'If you'd only been sent to me I could have saved your fertility.'"
It left her with depression and PTSD, which she still lives with seven years on.
"I could take all the physical stuff. I could take even that I might die but when something's taken away and it's not your choice, that's what I find quite stressful.
"If you take that person's choice away, it's like saying you're not worth picking for yourself what you want for your future."
The team followed 469 patients with various types of cancer at one referral centre in Malaysia.
They tested them for PTSD after six months and then again four years after they'd been diagnosed. At six months, 21% had PTSD. This dropped to 6% four years on.
"Many cancer patients believe they need to adopt a 'warrior mentality', and remain positive and optimistic from diagnosis through treatment to stand a better chance of beating their cancer," said the study's lead author, Caryn Mei Hsien Chan.
"To these patients, seeking help for the emotional issues they face is akin to admitting weakness.
"There needs to be greater awareness that there is nothing wrong with getting help to manage the emotional upheaval - particularly depression, anxiety, and PTSD post-cancer."
While the association between PTSD and cancer hasn't been studied in the UK, government data shows one in five people with cancer report having moderate to severe mental health issues. Macmillan Cancer Support estimates this to be about 530,000 people with cancer in the UK.
Living in fear
Dr Chan also stressed that many patients lived in fear that their cancer may return.
The fear and depression can cause them to skip appointments as they trigger negative memories, which can be detrimental to their health.
The study also found patients with breast cancer - who received special dedicated support and counselling at the centre studied - were almost four times less likely to develop PTSD in the short term.
Dr Chan said: "We need psychological evaluation and support services for patients with cancer at an initial stage and at continued follow-ups because psychological well-being and mental health - and by extension, quality of life - are just as important as physical health."
Dany Bell from Macmillan Cancer Support said: "It is tragic, but sadly not surprising, that so many people with cancer suffer from PTSD.
"While a common perception is that people should feel 'lucky' to have survived cancer, we often hear from people who felt that the support they received 'dropped away' when their treatment ended. The health and care system has a long way to go in terms of supporting people after cancer treatment."
Becki couldn't agree more. She said her mental health support was inadequate and the consequences remain with her.
"You see children and you try and block it out but realise the depression is something that will keep popping up," she said.
Her cancer is now in remission but she stills lives in a lot of pain that prevents her from working.
She devotes time to campaigning for awareness for fertility preservation, to make sure others don't have to go through what she has.
Becki said: "That's given me the empowerment to deal with it a lot better than if I'd been sitting at home, thinking about it.
"I'll keep raising the issue to make sure young people get their needs met."