Thanks everyone for the advice, I'm feeling better about the situation
I mentioned the issue to one friend who is laureled for performing arts and doing crafty stuff, she thinks the whole situation is nuts and it should be a no brainer that I needed to put taking care of my back and healing first, she didn't really have an opinion one way or the other on if it's best to formally end the mentorship thing or not, since it's not really an official acknowledged relationship
the way that an apprenticeship would be, she's more of the opinion that I should do whatever makes me happy and brings me the least amount of drama.
I was at an event in Beverly's area yesterday because another friend asked me to teach a class down there. And Beverly and I just sort of did the polite smile and wave thing and otherwise avoided each other, she even ignored me when I walked over to say hi. So I think ending the mentorship thing has taken care of itself and I don't need to do anything further.
We had actually been friends, not terribly close because we live far away and don't see each other much, or at least friendly acquiantances, for about nine years since I started being really active in the SCA. But it seems like the last two years doing big competitions has brought out a completely different and unkind side to her. I feel kind of bad for her in some ways, she really didn't look happy at all at the event yesterday, but also think the friendship is sadly probably not salvageable because I can't ever really feel comfortable committing to doing anything with her again in case I get sick or injured and end up unable to do it and she gets mad at me.