My advice- I'm not a pro, but have BEEN THERE.
Talk to somebody. Just plain old talk therapy helps. It's hard to understand, but having somebody there whose JOB it is to listen without judgment really helps. I did this (over much personal protestation that I "didn't need no stinkin' shrink to help me sort things out") my junior year of college to try to get my head screwed on right during a particularly dark period. It helps- furthermore, nobody has to know but you. Ask your doctor for a referral to a therapist and GO for a few weeks. That person will be also better able to recommend whether you need any kind of chemical intervention- I would tend to say "no" to drugs, as I've seen way more people get messed up further by them than helped. Just read another post of yours. If you have any kind of decent insurance, a mental health professional you see after referral by your primary care doc is just another copay. In the case of me and my wife, specialist copays are $40 instead of $25, but that's worth it if you need it.
Exercise. Get up and go lift weights for a few minutes or go for a run or whatever. Do this every day, plan on about an hour. Sounds like a lot, but the time goes by fast, and the benefits to your mental state are real. Your physical body's well being is reflected in your mental health, and as mentioned by others, the increased possibility of attention from the ladies and/or improved physical appearance will help. This is my ongoing plan for mental health. When I get to feeling really bad (my wife helps me notice, usually before I do), I ramp up the intensity/frequency of exercise for a little while and it usually helps keep me on an even keel. I come from a long and documented background of depression (both sides of the fam), so I just take it as part of being me, and something I have to deal with. I usually manage to stay pretty happy in spite of that, for whatever that's worth.
Being sad is part of being human, and when a relationship fails, it's natural to have some time where you examine what went wrong, mourn the loss, etc., but let me say this. I don't know you from Adam, but sounds like you're A) young, B) educated, and C) free to do whatever the heck you want to do. Take advantage of that in whatever you see fit to do. Another addition (I have been typing this as I read the thread)- this girl is 20. She's got some growing up to do- not a slam on her or you, it's just fact. Maybe I'm wrong, and she's incredibly mature and knows what she wants out of life, etc., but most 20 year olds I have met aren't ready for huge life-altering commitment. College/ Military service is great for letting people grow up in a relatively controlled environment where they can't screw anything up too bad. Let her do it, you don't need to stick around for that. If you need closure, fine, some people do. But if you take this girl back, you're only prolonging your own misery in the long run for some short term comfort.
And although I have rambled enough, here's my personal philosophy. Somebody smarter than me probably said it first, but here goes anyway. Choose happiness. It's a decision, make it. :)