Morag McTiernan was 17 when she first suffered symptoms and she believes her young age led her GP to dismiss breast cancer as the cause.
Morag, who is now 21, said: "I can't help but think that if I had been an older lady who had the same symptoms my GP would have thought about cancer.
"There were two years between the pain starting and eventually being diagnosed.
"Who knows how much the cancer could have grown in that time, I might have been able to keep my breast if it was diagnosed sooner."
Breast cancer is incredibly rare in teenagers, and only four are diagnosed with it in the UK each year – a chance of one in half a million.
Morag, from Middlesbrough, first went to her GP in 2010 because of pain in her right breast as well as discharge, but she was told that it was an infection of the milk duct.
The pain eventually became so bad that the teenager, who was studying performing arts and was a keen ballet and contemporary dancer at the time, went to get a second opinion.
She said: "I couldn't dance and I kept just thinking, they said this was normal, it's nothing serious. But I was wrong.
"I went to a different GP and they put me at ease, explained it was probably nothing to worry about but referred me to a specialist breast clinic for tests."
Two days later Morag was told she had breast cancer. She was just 19.
She said: "I should have been more shocked, but I was kind of expecting it, I knew something was really wrong.
"I was referred to the Royal Victoria Hospital in Newcastle where they have a teenage cancer unit for a lumpectomy.
"But later that week I was told that the cancer was bigger than they had expected and so I had to have a mastectomy.
"It was horrible news, I would have done anything to avoid that surgery. I was dead against it from the start, but I knew that the surgeon knew best.
"The operation did take away the cancer and that's what's important. I have had the breast reconstructed and had both things done during one operation so when I woke up it was all done."
Morag has recovered from her surgery and has also undergone radiotherapy and hormone treatment.
She's looking forward to returning to university in Sunderland this month, but said her attitude to life has changed.
She said: "I don't make plans any more, you literally never know what life is going to throw at you. The cancer has definitely made me a stronger person and it has taught me to accept whatever life has in store for me."
Her parents, Mhairi, 53, and Peter, 57, are running the Great North Run this weekend to thank the North East's Teenage Cancer Trust for its support in looking after their daughter.
Dr Mark Verrill, a consultant medical oncologist at the Freeman Hospital in Newcastle, said: "Breast cancer in young women is exceptionally rare. The chance of being diagnosed in this age bracket is one in 500,000."