Hi Jane, I'm not on the other side of the coin yet, but made the decision today for a double mastectomy. I have a rare lolbular cancer that is very aggressive. I have a good risk of it affecting the non diseased breast. I am only 50 and I am a worry wart so in conjunction with my doctor I asked him to do a double mastectomy and he agreed and supported my decision. I also made it because i am a big breasted woman (size E) and I have a friend who has been a qualified prosthesis fitter for 30 years and we talked about women she's helped over the years, the problems they faced as larger breasted women with back and muscle imbalance issues due to a remaining heavy breast and how most of them wished they'd had the second one removed at the time of the first mastectomy.
This journey is yours and only you will know what you are comfortable with. But for me, while its taken me probably 8 weeks to get to this one, I am confident that for me, a double mastectomy is the right decision.
I wish you all the best.