Sorry to sound harsh, but dump her. I know it is hard, but sometimes it is just better to cut all ties. It sounds like she has sucked you dry enotionally, and now it is time to take care of YOURSELF.
Whilst I am the first to say that friendships do take a lot of work and effort on both sides, sometimes, if a friendship is not making you happy, the kindest thing to yourself is to let go.
No way would I want that type of person as a bridesmaid.
I wish you all the best. It is such a toughie, you really have my empathy.