Abi Rounds - who had watched her own mother and aunt fight the disease – told her loved ones she would do everything it takes to beat the disease.
And she added defiantly that she would 'kick cancer's butt'.
Mum-of-one Abi, who's now 23, explained: "When they told me I had cancer It was terrifying. I I felt sick and the worst went through my head.
"As soon as I came home from hospital, I got my two-year-old daughter from my sister who was looking after her, and got my cousin and sister to come to my house and I told them the news.
"But from there on I thought that's it, I'm going to do this - nothing is going to stop me now.
"From that moment I've got on well, I'm really positive and the support has been overwhelming.
"People around me knew there was something wrong and it was getting to the point where I was having to repeat myself - it was too much."
Abi, from Leominster, Herefordshire, added: "I thought maybe if I do a video I can get my message across.
"It was probably one of the hardest things I have ever done, but looking back it is one of the best things too.
"The support has been absolutely amazing and has really got me through the tough times.
"It has been incredible getting messages from strangers telling me to keep strong."
Abi – whose mum had a single mastectomy when she was 46 - had been watching TV when she felt an itch on her right breast and found two lumps, which were found to be cancerous.
In the video - which has been viewed more than 10,000 times, Abi said: "The reason I am posting this is because I have had so many lovely kind messages off so many people."
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She adds: "I just thought instead of me repeating myself to everyone, this would be the easiest way for me to say it, and you will all find out sooner or later.
"I never thought I would be sitting in the same position as my mum, at the age of 22, getting told I have breast cancer.
"It is scary, and it is a shock, and you just don't think it will ever happen to you.
"But with the support of my amazing family, fantastic friends, and my daughter, I will do that and I will kick cancer's butt!"
After a family Christmas with partner Jamie, 23, and daughter Ruby May, two, she had a double mastectomy in a four hour operation on January 13.
However further tests have revealed the cancer has spread to her lymph nodes, so she is now undergoing chemotherapy.
Abi said: "I'm doing really well and I just want people to know that you can have cancer at this young age, and always just go and get checked."
Abi and her friends have has also raised more than £2,000 for Breast Cancer UK by shaving their heads.
To donate to her cause, visit Justgiving.com/abigialjeanrounds.