Yep you just need to tell her as difficult as that can be, could one of your other bridesmaids help you out with this perhaps? I know with my own experience I had a bridesmaid who became a real pain and I didn't really want her anymore or know how to tell and also found out she was *****ing about me behind my back anyway, so one of my other bridesmaids spoke to her and suggested "look your obviously not happy with being in the wedding why dont you let (me) find someone else", this then led to her *****ing "oh what she doesn't want me anymore is that what she said" and my bridesmaid said "No but thats what I am suggesting as I dont think either of you are happy". This then gave her an out and she opted out I enlisted another friend who I would have much rathered and it was all good, and the other girl still came to the wedding no fuss and it was all good.
Best of luck with your situation and I really hope you can sort it all out.