Tara Jones, 26, had the mark removed three years ago and, after a biopsy, was told that the growth was harmless.
But over the next few months her health deteriorated, she lost two stone and started passing out.
Doctors put her condition down to depression, told her the weight loss was anorexia and dosed her with anti-depressants.
It wasn't until the mother-of-two went back for more tests earlier this year that medics finally admitted they had made a huge mistake.
The cancer has now spread to Tara's bones and she has been told she only has months to live, Daily Mail reports.
Jones said, her voice quavering, 'I don't want to go to heaven and watch over them - I want to be here with them.
'I just don't know how they will manage without me. I have never spent a day apart from them.
'I will miss everything I do with them so much - taking them to the park and feeding the ducks, digging up worms with Tyler.'
Doctors first noticed the mole when Tara, from South Wales, went into hospital to give birth to her second child in 2005.
She had a biopsy at the Royal Glamorgan Hospital and was given the all-clear.
The former trainee lifeguard was only persuaded to get a second opinion after a friend, whose mother had just died of cancer, recognised some of her symptoms.
She went for more tests and a doctor from the hospital visited her at home to break the news.
Tara, who lives with her partner Chris Davies, said: 'He told me there had been a terrible mistake. The mole I'd had removed in 2005 had been cancerous all along.
'I was screaming and crying and told them they had ruined my life. But it's too late for me, there's nothing anyone can do.
'I'm determined to enjoy every minute I have left but it's hard.
'It's one thing knowing you are dying of cancer - but it is hard not to be bitter when it could so easily have been stopped from spreading if the test on the mole had been done properly.
"I am dying because someone made a terrible mistake.'
Tara's partner Chris, 29, has now given up work as a building contractor to look after her and the children.
The family has also appointed a solicitor to fight a case of medical negligence.
'It is beyond belief the way Tara has been treated. I love her so much and I am just staying strong for her and our kids, he said.
'A doctor's mistake has completely shattered my family.'
Cwm Taf NHS Trust, which runs the hospital, has now admitted medics made a mistake and promised financial compensation for her family.
A statement from the Cwm Taf NHS Trust, which runs the hospital, said: 'We are able to confirm the Trust's involvement in a claim in the delay of diagnosis of cancer.
'But unfortunately the Trust is bound by constraints of confidentiality and cannot comment further at this time.'