OK so I've been best friends with this girl for almost 2 years now, and in that time, we've became unbelievably close. We hung out in November of 2012 and after I got home she said she liked me. A little bit passed and I started liking her too. So I asked her out one day and she gave me the typical "we're too good of friends and i don't wanna lose you." so about a week passed and I asked her out again. And she straight up ignored me for about a week. Wouldn't talk to me, wouldn't text, wouldn't do anything. So finally she started talking to me again and about 2 weeks later I asked her out again. And on Superbowl Sunday, February 3rd, 2013, she finally said yes. We dated for almost 2 months and then she broke up with me. She confessed that she was in a "suicidal phase" and that she was cutting as she texted me (shes not anywhere near that phase now. She used to cut now she's over a year clean. She's actually a really happy person now) and she said having a boyfriend just adds worries. About 3 weeks later she dated another guy for a week but came to me and said I miss you, I feel like he's only dating me to get something from me. You like me for me. And I want that back. So we started dating again and then, after only a week, she broke up with me. I still wanted our friendship, so I started talking to her again. It took a little, but we got back to normal talking. Now I moved on for a while dating other girls, and whenever I dated the other girls, me and her would talk once a week or something and after about a month, we weren't talking at all. She came up to me later and told me, "i need to get this off my chest. we were such good friends before and we could go to each other about everything. you said you'd always be there for me, but your not anymore, and i don't want that to be gone." so we've been talking ever since, and I'm pretty sure she wants the friendship to go on, but I'm not sure if I should let it. I mean I kind of miss her, but she talks to about 800000000000000000000000000000000000 other guys, but I just wonder if I'm even relevant to her anymore. I don't know I might of left some things out but if I did ill get back to you guys.

If you can be just friends do it... if not just tell her you can't.. and why. Its not going to change how she feels but she could at least respect the honesty.

So you are one of 8000000000000000000000000000000000000000000 other guys. So what? Why is she still the only ONE for you? Stop wondering if she still thinks of you and what order between all those other guys are you. Go back to the fun life you had before she complicated it and took it over. Or you allowed her to take it over would be more accurate.

Hey buddy first of all keep your chin up and second of all this girl and I will be as bland and sympathetic as I can be... she is an attention seeker. She gets bored very quickly or can't find a scenario to make it about her or have central focus only on her so she only goes out with people for a week gets bored and goes off to the next. In regards to the self-harm, have you witnessed her doing it? If you have, you should have told someone about it so she can seek medical help, if you haven't is she only saying it so you feel sorry for her? Or is she really self-harming herself so you stay by her side this is all psychological and she really needs to sit down and see a doctor or counsellor... regardless you need to tell a parent or teacher someone you can trust because self-harm is a serious issue.
Furthermore, take a look back did she ever worry about her looks, what people thought of her or if anyone talks about her? Did she ever ask you questions where you always had to reassure her? Did you always have to do things to keep her happy?
The other thing is not sure of what your age range is but she could possibly just want sexual attention, she gets it from other guys and comes back to you because she knows your not like that and she might feel guilty or disgusted by herself so keeping you someone as you said that she said "likes me for me" makes her feel better and not think about sleeping with the other guys as a disgusting thing.
It's sort of a toxic friendship, the moment you find a girlfriend she'll be jealous and angry and think you're not her friend anymore and you don't make the effort to be friends with her, which then will make you feel like crap and to please her and keep the friendship going you'll find yourself alone for a very long time.
The question is do you want this friendship to continue? You have two choices, either stay in this toxic friendship and risk not been able to have a relationship with any other girl or two set some boundaries and rules and if she's a true friend she'll understand.