No More Mr Nice Guy is a great recommendation in my perspective. There is also a forum for men with the nice guy syndrome:
No More Mr. Nice Guy Online Support Group - Powered by vBulletin
Also, check out something called Assertive Communication. It's a style of communication that allows you to take care of your needs, confront conflict and be honest, in a way that's respectful toward others.
Bottom line, I think you need to start addressing some conflicts head on, instead of avoiding them. Once you get used to the fact that conflicts are a natural part of life and they will always be there, it gets easier.