Hi op,
I'm always late to the discussion so if you don't read this, well, maybe someone else will....
I just wanted to say I understand your situation and why you feel uncomfortable.
If it helps, may I offer up a little bit of insight that I went through?
When I was a teenager I went to visit family in Germany, and I was really taken aback when the family brought me to a beach and all the women started taking their tops off. It wasn't a topless beach, but it's allowed, and my female cousins who were in their late teens, my aunt's, they all started taking off their tops for some sun. And here I am, 16, and I've never seen anyone elses breast before, having grown up in a relatively conservative environment.
It didn't shock me, but I was unprepared, and I was not forced to participate, (my younger female cousins didn't take off their tops either) but the thing that shocked me the most was the fact that nobody was paying any attention.
I thought for sure that all the guys would be all eyes, and they 100% weren't.
So, in a spa that everyone else is going to be nude, yes you're always going to run across some people who are new to this and self-conscious and uncomfortable and I wouldn't blame you because I've actually never been to anywhere that in public, full or even partial nudity was allowed/expected / required but now, in my ripe old fourth decade, I believe that I would participate in something like that because I believe that no one's really looking, no one really cares, no one's really judging you, and that we have to be more comfortable inside our own skin. I understand your reasons for being extra uncomfortable, but I'll be honest with you, to me, women who have gone through mastectomy, I know they're typically more self-conscious but I believe that that's a badge of incredible courage.
You've gone through something so emotionally and physically difficult, you've done it in order to preserve your life/health and it was an incredibly difficult thing that you've been through and I have extreme respect.
Hope that doesn't make you feel more self-conscious.