First off, I'm a woman and what she did to you was completely reprehensible. It's such a cold thing to do.
Based on your post, it's not your fault that she left. Harming yourself is like punishing yourself for something over which you have no control. I've beaten myself up emotionally many times over things over which I had no control. If this makes any sense, by harming yourself you're giving her the power to ruin your life. I know I can ruin my life on my own--I don't need to give anyone else the opportunity.
Shortster is right--go to a health professional and get some help right away. Your life is too raw right now to know what direction to follow.
My suggestions:
1. Cut her out of your life. Block her cell phone calls, have an answering machine pick up your calls (or arrange a code with your friends, like ring twice, hang up and call back). I know it sounds surgical but let's face it, she's a habit you took up and now must drop. Habits are hard to break but they can be broken. Get rid of everything she ever gave you. Even the tiniest thing. You'd be surprised how enormously important that tiny thing can become.
2. If you can, stay away from mutual friends for awhile. If you run into them, they may feel the need to tell you what she's doing. They may feel they're doing you a favor but they're not. If they bring up her name, change the subject. If they insist then politely excuse yourself.
3. If you can, try to start one of your school assignments. Is there a group you can join to work on it? I'm thinking about an assignment where you might have to do a critical analysis of a particular author's work. Being in a group is like being in a book club--you read the work and discuss it.
If all of the assignments are of the kind where you have to do them on your own (science stuff is like that, I speak from 30 years' experience), then get out of the house/dorm/apartment and go to the library. Perhaps a change of scenery might be the thing.
4. Gather your support network around you. As Shortster posted, you can never have enough friends in times like these. They'll stick by you and may give you some perspective that we can't give you.
5. It's completely OK to mourn the loss of a relationship. If you have to cry, then cry. You have to get rid of all the hurt, but it won't leave you all at once.
6. The insomnia is perfectly in tune with the depression (speaking from personal experience). You might be able to find a med that will help you in the short term. Nothing is worse than being awake at 0200 with no one to talk to. I still wake up at 0200 (instead of 0445) and nothing is worse than lying in bed, wide awake, hoping that I'll get back to sleep soon, instead at 0430).
7. Don't be surprised if you have dreams about her. When my First True Love showed the back of himself I don't know how many times I dreamed about him (don't have the diary I kept in front of me so I can't tell you what the dreams were about) and woke up in a cold sweat. It's been almost 26.5 years since it happened (that should give you a clue to my age), and I still dream about him (very rarely). These "new" dreams all have the same theme--he realizes he has made a mistake and wants me back. I keep telling him I'm married (which I am) and there would be no way I would ever go back to him. This will happen to you at some point.
8. I don't think guys do this, but consider keeping a journal to which you can pour out all your feelings. I did and I'm glad. I haven't read it in dog's years but looking back on it I see incremental progress. You don't realize how far you've come until you look back.
9. If you can, please try and find one good thing every day that makes you happy. A pretty sunrise, a flower, something you hear on the radio (today I heard a great phrase from a song, "chasing pavement that takes me nowhere"--how cool is that?), a nice cuppa, just some little thing. If you find even the smallest bit of happiness in an ocean of sorrow, you are going to be fine. It took me at least six to nine months to work through all the issues and I still bear the emotional scars.
10. Since she left under less-than-friendly circs, I wouldn't count on getting the brass from her. If you have something like a small-claims court you could sue to recover the amount, but you'd probably have to have some kind of documentation to prove she owes you the money. Otherwise it's a "he-said/she-said" thing and that won't hold up in court.
11. Now is the time to become comfortable in your own skin. Be a friend to yourself. If you can't accept yourself for who you are, you'll never be able to accept anyone else for who s/he is.
12. Please keep posting so we know how you are doing. You're welcome to pm me if you need a shoulder to cry on.