She's not in your life anymore, right? Before, your anger served a purpose...keeping a dangerously jealous person away from you. But she's gone away now, and turned into someone less defiant, right? So your anger, or bitterness won't help you in this situation at all. If you stay mad, or upset... your friend could be out there having a good time. The only one you hurt by holding onto being upset is yourself. I'm sorry to hear you had such a bad experience with someone you might love, but I'd say you should forgive her. Then, if this person possible reenters your life, you can start new. Forgetting the grudge you had because she changed from someone you respected into someone you couldn't. It is the worst feeling when you see someone you thought so much of, have no respect for themselves... it's irritating you want to slap the sense into them, but you cannot. You just have to watch life either make them stronger, or take the fight out of them, make them conform. Her jealousy was her responsibility, but her changing, that's something that life does. It was not your fault. Her being jealous of your accomplishments, doesn't sound like a friend who truly wanted what was best for you. A real friend wants you to succeed. So it might be best that this girl was removed from your life. In the future, I'd caution you to stay away from people who trample your goals to try and further themselves, or consider your success to be a threat to their own. But in the end, if you can forgive her mistakes, you will feel free of the past, and it hopefully won't bog you down.

The thing is, I really do have alot to loose by offering her forgiveness.
I know the way she is, and if I talk to her again, truely forgive her, she'll just start... being her again.  At the same time, who am I to deny her freedom from herself?  Who am I to deny her a way out from her past?
I've given her so many chances in the past, and she's always taken them and used them against me, taking one thing or another... but... there's always the chance that she's changed. 
Or... she's just become a monstrosity of herself, none of the strength of before, but still the underhanded manipulation and bully-ing.   I think I'm strong enough to take her if she is, but I can never be sure.
I feel no anger twords her, that'd keep me safe, I only feel regret and longing for her to tell me all the cruel things she said were lies.
That's not a good state to talk to someone.

Thank you for your advice, I think I'm going to forgive her, dispite these things.  Maybe it'll heal, but... somehow I'm dubious it will bring anything but pain.

While I've been spared of such difficult situations like yours, I've witnessed some second hand. There is a great difference in forgiveness and pretending everything is like before. While I agree that it pays to at least try forgiveness, it's important to make a point of not wanting to get back to relationship with her. My suggestion is to forgive, but not forget. It will propably take awhile before you can decide whether you still want to be her friend, even if you've forgiven her past wrongs and, as I believe you well know, much longer for anything else, if either one of you still has some hopes - that, incidentally is good to check with her, as well.

Just my 2 eurocents...

Forgiving doesn't mean forgetting and it doesn't mean pretending that things were the way they were before. If she's not a part of your life now, she doesn't have to become a part of it again. There's really no way to know how she feels now unless you ask her, but if having resentment against her is hurting you, then by all means let it go - perhaps it's best if nothing changes between you, perhaps not. But forgiving her will help you, if nothing else. Even if you don't speak to her after telling her.