She might feel she doesn't "deserve" you, that it's draining her to try and be a "proper" person.
You need to let her know you will be as much help as you possibly can, either as a friend or a boyfriend. You are probably the only person who knows she hurts herself. Now she can't face the reality of dealing with herself, so she denies it, and finishes things with you... do you see a pattern? She can't cope with pain, with negative feelings at all. You can't teach her that either.
This girl is seriously ill. She needs a lot of care, and attention and help from a PROFESSIONAL. And she needs to gently realise that unless she gets it, she is killing herself. She needs support, but she needs to accept that support first. There is nothing you can do to convince her to let you in.
Talk to her gently, and leave it with her to decide. She has a chance of a friend, support and a chance to heal. But she has to make that choice.
Look after yourself, you're really putting yourself through the mill.

:'-( well that's your choice not mine

You cannot help anyone who is not willing to help themselves! I would say go low and talk to her parents or someone close to her. If not then I say make her go see a doctor. Trick her into it just so a doctor has the chance to tell her to her face that she is sick!
It's the same concept as anorexia or somehting like that! I have anorexia and denied I had a problem until my boyfriend took extremely extensive measures.
If there is nothing you can do anymore, leave her. I know its hard but sometimes you have to do what feels like the most wrong thing ever!