Lord Trilby, welcome to the world of love/pain. It's a trial by fire, no two ways about it.
I've been there many times and I feel your pain.
Hang in there, because it does get better. I won't bore you with the details of my previous heartbreaks.
If it helps, some things which, odd as they may sound, I think you will agree with in a few years:
1. She's not evil, but her feminine psyche is just experimenting with how to manipulate men. She is secretly horrified but at the same time thrilled at her behavior...they all do it, some more than others. It sounds horribly cruel but it's just human nature, you absolutely should not take it personally. You need to recognise that what is driving you to keep going back is, to put it bluntly, your balls, or more specifically, what they contain, if you know what I mean. However you as a human hold the power of rational thought and can make a conscious choice to walk away.
The more you keep coming back to her to try to patch things up, the worse it will get. (men do it too so I'm not trying to be misogynistic here)
2. You should sever all ties with this girl and her friends, etc. until you have no strong feelings towards her one way or the other. If you share friends, take some time off from your social circle. Do it cold turkey and remember, you don't owe anyone an explanation...but at the same time don't necessarily burn bridges with people, just walk away for a bit.
3. You will look back in a few years time and the only thing you'll feel bad about is maybe that you posted about it online, if even that.
In the meantime find something wholesome to occupy your mind, not drugs or alcohol; as with previous posters I can attest to the fact that they only make things worse.
Best of luck with it