You are going to feel like shit for a while. Period.
There is no way around it. Come to accept that fact and then you can work on getting through it.
Take some time to mourn the passing of something that made you feel good. Grab a drink, watch a series of sad/breakup movies (I suggest Swingers and Pixar) generally indulge for a day or two. Do whatever you feel like doing short of contacting the ex. This is going to be a vacation from that. Even if in the future you guys are going to be on friendly terms now is not the time for that. Both of you need space to start thinking straight.
The real healing takes time. In the meantime, while it feels like there's a hole in your chest, keep yourself busy. Seriously I don't want you to think this means pick up a hobby for a day a week or some shit like that. Get busy. If you have the time to think about anything else besides what to do next you're doing it wrong. Go to the gym, play music, read a book if you can, whatever.
The point of this is to get perspective. Its a mindbogglingly huge world out there with more possibilities than we will ever know. Your brain is making you feel like shit. It's all in your head. Give your brain some stuff to chew on besides past regret. If you don't think of other things your mind is going to constantly throw what if scenarios at you. Anyone whose gone through a breakup can tell you the what ifs are probably the worst part. What if she's sleeping with some other guy. What if I would have done x different. What did she mean by that?
Over analyzing is your enemy and you wont be able to help it by just sitting there unless your the goddamn buddah. You are a uniquely beautiful person. Someone loved you and I know that many others given the chance will too.
Give yourself some attention. Work on yourself.
Also sometimes there'll be the point where youll have to whoop yourself in the ass. It's easy to sit and look at the pieces when somebody breaks you. Nobody is going to be there to tell you to get off your ass and pick up the pieces besides yourself. Be your own drill instructor for a bit and build something from those broken bits.