Winter this is such a great thread.... unfortunately I don't feel I've been particularly graceful in my dealings with others lately so not sure I can contribute much of use.
In some cases I try to remind myself that others press our buttons because they remind us of what we don't like in ourselves. I have found this a useful thought, especially when I take a strong dislike to someone for no good reason - remembering this I have managed to get over it completely. It's a whole lot harder when we have complicated family relationships with people and struggle to figure out what is a genuine personality clash and what comes from manipulation, jealousy, etc.
Anyway I hope you find a way to hold your head up with your MIL. Try to remember that if she is trying to bring you down its because she feels second best. Maybe that will help you stay calm and detached.