Ending a friendship without good cause is difficult! However, this relationship seems to be more of a user-usee relationship. I.e. she is using using you for her purposes and you are being used! And, not enjoying it!
Assuming that you do not have a lot of mutual friends, therefore she can't easily "poison the pool", I would suggest that you write her a letter setting forth your complaints [great detail is not needed, but accurate descriptions of what you perceive are encouraged] and informing her that henceforth you would prefer to have no contact with her.
This may generate any of several replies:
1) OMG, I have offended and I never meant to. I was being so selfish, etc. [This may be genuine or not.]
2) You cold b_tch! How could you say such things to me? You are no better than a dustbin and I was trying to be your friend.
3)What have I ever done to you that you would treat poor me so cruelly?
At this point you have to ask yourself if there is anything about this "friendship" that is worth saving. Judging by your post, I would say not and just shrug it off and get on with life. If she persists tell her that NO CONTACT means just that.
I have done this on occasion in my life. It is painful for both parties early on, but I preserved my sanity and self esteem by refusing to be a doormat for someone who wasn't really a friend at all!