I recently had a very similar situation: a serious problem between me and a close friend. I knew that there was only one solution, which was to talk to him openly about it. But I was afraid of hurting him, and I was afraid of the pain I would feel due to his reaction, which I knew would be really, really bad.
I went around asking what I should do, but I knew what I had to do. There was just no way around it, no nice way to say it, but it had to be done. Maybe you’re not like me at all, but it’s possible that you know what you need to do and you just don’t want to do it. No judgment about that, by the way. There’s nothing wrong with not wanting to cause pain, or feel pain. It’s just that sometimes, there’s no non-painful solution. Either you’re going to talk to her point-blank and have a certain kind of pain from hurting her feelings and enduring her response, or you’re going to do nothing, and have a completely different kind of pain because she won’t leave you alone.
While it’s true that she’s emotionally manipulating you and blackmailing you, not to mention harassing you, she’s not doing any of that on purpose. She is trying to fulfill her own needs, which are overpowering her recognition of your needs. We all do it in some way or another; some of us cope with it better than others, and some of us are lucky enough not to have such desperate emotional needs.
Here’s the nicest way to talk to her, and for that matter, to anyone with whom you are having difficulty. Start with compassion. For her: she’s wounded, and she’s responding to her wounds in the only way she knows how. Also have compassion for yourself: her responses to her wounds are causing you quite a bit of emotional turmoil. Recognize your responsibility for fulfilling her emotional needs: none. You never signed on to bear someone else’s emotional burden, and there’s nothing in Life’s Big Book of Rules that says you have to. Recognize your competence for fulfilling her emotional needs: none. You’re not a therapist. You can’t help her any more than you could help her with a broken arm. Recognize (and respect) your own needs. Life is way too short for you to let someone diminish the quality of yours.
When (if) you talk to her, don’t use any labels. Don’t tell her she’s too needy, or she’s being intrusive. Only describe the behavior that’s bothering you: she is messaging you too often and she won’t take “no” for an answer. Very specific. No generalities like, “We’re getting too close,” or “I didn’t plan to renew our friendship.” Basically, nothing she would argue with, no opinions, just facts. Also, own your feelings: don’t tell her she’s doing something wrong. Someone else might really welcome her, perhaps someone who is lonely. So, it’s not that she’s wrong to treat you this way; it’s that you feel uncomfortable with these specific behaviors of hers.
It’s important to note that she would not be able to manipulate and blackmail you if you didn’t let her. Again, no judgment, it happens to everyone. The main reason to remember this is that when (if) you have this talk with her, you must steel yourself for her reaction, which will probably involve more attempts at manipulation and blackmail. You must not let her. No matter what horrible thing she says about you, you have to keep your mouth closed. Don’t defend yourself, don’t make excuses, don’t let her manipulate you into turning it into a discussion, don’t let her demand that you justify yourself to her satisfaction. You have your own needs and your own reasons; if you feel justified, then never mind someone else’s standards of justification. The best answer to any “Why?” is “I’m not comfortable with it,” and leave it at that.
If she gets really upset and hurt, then comfort her. But only comfort; don’t offer any concessions, don’t offer to change yourself in any way. You have friends who’ve had bad breakups, or who have been mistreated, or who have simply had a painful situation arise in their lives. Comfort your needy friend the same way, that is, her pain has nothing to do with you, and you just feel bad for her because she’s hurting.
That’s the nicest way: straight talk about specific behaviors, compassion for both her and yourself, don’t make any concessions. Best of luck to you both.