A mother said she has been "robbed of her life" after her cancer was not spotted and was now terminal.
Justine Jianikos, 30, from Pontypool, Torfaen, has an aggressive form of breast cancer which spread to her spine and lungs.
She is writing cards for her three young daughters for milestones she will miss - birthdays, weddings and passing their driving tests.
Aneurin Bevan University Health Board said it was investigating.
"Even though I am not going to be here for them, they need to have something to know I was always thinking of them," she said of Josie, nine, Halle, six and Ruby, five.
Ms Jianikos said she was now facing the "very daunting, horrible prospect" of not seeing her children grow up.
'Mistakes and delays'
"I might not have deserved this but they certainly didn't deserve it," she added.
"They certainly didn't deserve to have their mother taken away from them for mistakes, delays, and something if it was caught a lot sooner could have been treated a lot differently."
Ms Jianikos said she first noticed a lump near her armpit last May and said she was initially told it was a fibroademona - a fatty tissue lump that commonly occurs in the breast.
It grew to about 5cm in diameter and by September had become painful so she returned to her GP.
"This time it took three months for me to be seen in the breast clinic," she said.
"After receiving a copy of my medical notes, we noticed it was stated twice - in bold, 'urgent, suspected cancer' - and it still took three months."
In December she was offered a biopsy - something she was told was not routinely offered to women under 30 - and was diagnosed with breast cancer four days before Christmas.
Had these tests been carried out months earlier it could have "saved" her life, she said.
"If only a biopsy was taken it would have shown that there was cancer there, and my life could have been saved, you know," she added.
"And so that's why I'd say to anybody, after the experience I've gone through, 'don't always take the first word a doctor tells you, fight for your life - fight for your life' because I have to fight for what's left in my life. And I shouldn't have to."
When the biopsy results came back Ms Jianikos learnt it was a particularly aggressive form of the disease called triple negative breast cancer and was stage three. It is now stage four and has spread to her spine and lungs.
She said her oncologist believed if her cancer had been found earlier, the outlook would have been different.
Ms Jianikos said: "I believe I went into that appointment in May, a young, fit healthy girl, whereas I believe if it was six months prior to that I would have had a triple check like you should have which consists of a biopsy, a mammogram and an ultrasound to make sure it's not something sinister."
"I didn't have that. In my head I believe it's because I was young and because of the pandemic.
"I am now looking at a very different life. For the rest of my life, I am going to be fighting to hold on to my life. That is a very daunting prospect."
"I've told myself that as long as I can wake up and make my girls breakfast, as long as I can take them to school, as long as I can be a normal mum to them, that's all they need to know," Ms Jianikos said.
She has started her fifth round of chemotherapy and then has one more to go.
"People look forward to that day, they've come to the end, they get to ring the bell, it's a good day for a lot of people.
"For me, that is going to be the scariest day of my life because I know after then, there are no drugs in my body trying to fight off this cancer until my next treatment plan is drawn up."
Her family is trying to raise £100,000 for private drug treatment only licensed in the United States which could help prolong her life.
"I can't save myself but if I can save anyone else, by raising awareness of triple negative breast cancer or cancer in general," she added.
"If there's anything you are not sure about, go and get it checked out and trust your instinct - you know your own body."
An Aneurin Bevan health board spokesman said: "We are very sorry that Mrs Jianikos is not happy with the care that she has received.
"We are currently investigating her concerns and will keep her fully informed with the progress of the investigation."
Richard Pugh, head of partnerships for Macmillan Cancer Support in Wales, said the "sad reality" was that Ms Jianikos's "heart-breaking" story was not unique.
He estimated there were thousands of people across Wales receiving late cancer treatment because of the pandemic and urged the Welsh government to "get to grips with the backlog".
Mr Pugh urged anyone with possible symptoms to contact their GP.