If she had been saying similarly disparaging things about women or lawyers or the Inuit, would it have upset you this much? You might have been overly sensitive to her remarks as you were previously overweight. Maybe not. It doesn't really matter. What matters is that the trust is completely gone for you.

It's possible that you could get the trust back with time, effort, maybe counseling. It doesn't sound like you want to pursue that. That's okay if that's your choice.

Reading your post a second time, I wonder if this is more about how you feel about yourself. I wonder if you look back at the time when you were obese and think "It was fine, I had relationships, I was successful," but really, underneath that, you did not feel okay about it. Really, underneath that, you disliked who you were. When your fiance reviled overweight people, you heard her attacking you. Being attacked isn't great, but it is unbearable when you agree with the attacker's criticisms about yourself on some level, when they found your vulnerable part, the part where you question yourself. When your the rest of your support network (save your brother and his girlfriend) think you shouldn't leave your fiance with whom you've been happy and whom you've loved for five years, you seem to feel that they are taking her side against you. I think this is deep, primal stuff. I think you should give yourself some time to get out of fight or flight mode. Some time to calm down and get perspective.

Your mother and mutual friends don't have to agree with your decision, but they do have to respect that this is your decision. I don't think that you have ended your relationship with your mother and mutual friends at this time. I imagine they all still care about you.

To answer your question, I don't think you need to do anything right now. I think taking some space to get perspective is a great option.