The problem is you. Her leaving you has amplified your underlying insecurities and this has made you depressed. You're not in love with her. If I put a hotter, nicer chick in front of you right now ,who pushed your ego buttons the right way, you would forget all about this dingbat that just dumped you.
It's been discussed constantly, but you need to hear it again : At this point in your life (20), you should be working on yourself. Imagine the man you want to be. Is this chick part of the picture ? Imagine if you somehow "got her back" you could look at her and think "That's my girl ! The one who's into her ex more than me !"
Get some "plates" going and get out of this stupid one chick syndrome...this way, you are not really breaking up, instead you'll just be transitioning from spending more time w/ one chick as opposed to the other.
Let me tell you a story...when I was around your age, I imagined myself living in a certain neighborhood, driving a black sportscar, and having a bunch of girlfriends. Instead, I married a woman who didn't love me, got divorced, and went through some terrible depression. But guess what ? I realized I wasn't depressed over my ex, I was depressed about ME !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Today, over 20 years later, I LIVE IN THAT NEIGHBORHOOD, DRIVE A BLACK SPORTSCAR, AND I HAVE A BUNCH OF GIRLFRIENDS