How do I handle this?

I think you should try talking; she currently is the only obstacle in your friendship. Approach her calmly, let her know you just want to talk. Let her know that her fiance means a lot to you but that romantically nothing will ever happen. Let her know you are just missing your friend. Stress that you respect their relationship. Whatever she says to you respond calmly and rationally. If she brings up your past mental instability, admit it is a part of your past and say you're feeling much better. If you get angry with her she'll get angry as well. I hope it works out =]