Sir Eddie, I've got to say first and foremost, from one "nice guy" to another, you've got my condolences.
As has been stated above, you're a "nice guy". Well dude, I hate to tell you, but women in their 20's don't want a nice guy. They want a very specific kind of dude (I think you refered to them as "douchebags" in your post) that they can "fix" and "change" and make all their girlfriends jealous with. You and I are not that kind of guy! We've bestowed upon ourselves the dubious honour of being "nice". We're the guy that they shack up with for a while when they've been dicked around to the point where they feel so horribly low about themselves that they need that egocentric pick me up that comes in the form of our affection, consideration and "sensitivity to their needs". We're a tear-stained shoulder, a firm hug, and a rub on the back that says "Everything is going to be a-okay because you're a special, beautiful, wonderful person." That is our primary failure, as far as I'm concerned. Don't get me wrong, man. Being nice is fine. You just need to know where to draw the line. Women in their 20's get bored of that stuff though, dude. You'll compliment them, dote on them, make em feel like Queens. But, because you're nice, you'll give that up too easily, they'll get bored, and they'll start wanting more of a challenge. That challenge lies in the affections of scores of douchey, horrible dudes. They cut you loose, leave you feeling broken, sad and alone. And yes sir, it absolutely sucks, but that's the way that cookie crumbles. So it goes.
But for as hurt as you are right now, you're in a really, really great position. You're out of a stagnant relationship with an uninterested party, and now have the option to do something I don't think you've done for a while: Think exculsively about and act upon your own best interests. Granted, you can mope, and feel really terribly about losing someone who was special and close to you. Or, you can come to the realization that this person stopped wanting that closeness and willingly walked away from it. Now that this party is out of the picture, you've got yourself to get re-aquainted with. Do not call your ex. Do not speak to your ex. Do not honour your ex's memory with adolescent poetry and self-loathing. Don't try to be "the better person". Don't be the "nice guy who still values her friendship". That dude is a sap. The value of the two of you on friendly terms went out the window when she left you. Should you run into her, be civil. Definately be civil. But don't be nice.
As your sage-like, fellow TBers have offered above, there are many things that will fast-track you on the road to "what was her name again?" country. Hit the gym, lift some weights, start doing yourself the favour of engaging in activity that's gonna make you feel really good. Rediscover your pride, man. You've tried being a nice guy, and that blew up in your face. Now discover what it is that makes you a GOOD MAN. Find that, and pay it forward.
Hang out with your friends you've been neglecting, go bars, go to parties, take up some hobbies. Meet people. Hell, go out and meet interesting people. Meet funny people. Meet smart people. Meet sexy people. Surround yourself with interesting, funny, smart, sexy people, and make yourself available to them. I'm not talking soul-bearing, deep and meaningful, "I think we're connecting on a higher-level" type stuff, but just being cool, and being cordial, and being someone that people are gonna wanna be around.
Be a GOOD MAN, and take your pick from the cornucopia of ass that gets thrown your way.
And don't stop at just one. Get as much as you can. Be a gentleman. Be up front about your intentions. Be honest. But don't be subservient and cater to everyone's whims. Be good to yourself...
anyways, I forgot what I was talking about... I guess I just got a little too excited about the idea of a dude turning a rotten situation into an extremely advantageous one... either way, choice is yours. You wanna call her and be her friend, go ahead. I've got some Smiths records and a dark, moping room you can borrow... I just think that's a lame idea...
That just makes you scary. Just the girls that pass through your zone. Try it. Just a friendly greeting for now. 
Speaking of which, do not, I repeat, do not stop at #1 and think/say, "I have a girlfriend now." 
