I'm thinking of backing out as maid of honor of my friend's wedding
What is causing you to want to back out?
I'm in love with my friend's fiancee
Its expected that you try and be there for your friends wedding day, however if you are in love with her fiance then it may be a good thing. You dont want to be the friend who ruins the wedding because of these types of feelings. While it may be difficult, you are probably going to have to talk to your friend in private about this.
This is tough. I think I will have to do this though.
You are doing the right thing.    This is going to hurt her, but it would be much worse if you were to ruin the wedding or end the marriage.