@Pahill, you've probably given us the best news we could hope for, on your behalf. As @primek said, talk to a genetic counsellor about genetic testing, but at the endof the day, the decision to have preventative mastectomies is yours to make. I didn't have a mastectomy but I believe there is a closed faceboook group, called Reclaim Your Curves,that could be worth looking into.
AnAustralian website you may be interested in is Pink Hope, to look at pros and consof preventative mastectomies
Take care, & stay in touch