Hey, jumping on the bandwagon here, but I just went through a breakup too. Breaking up is weird- By the end we had a horrible relationship, and all I could think about was leaving. And then we finally had the last big fight and I packed my stuff and left the very next day. We were living together, we had pets together, all of it gone within 24 hours (though I suppose it had been building up for a while).
Now, even though I was miserable in our relationship, I miss him all the time. When we were together all I could do was focus on the negative things, but now that he's gone I can only think about the good stuff. Like, too little too late kind of a thing. He has a lot of hobbies so I think he's keeping himself distracted, or maybe has already moved on, but I moved cities, have no friends near by, and have never felt so alone in my life. I got drunk and texted him a few nights ago and he never responded. It was crushing.
I know that leaving was the right thing to do though, and if I had the chance I don't think I'd change anything, but damn if it's not the hardest fucking thing. But you know what's kind of comforting to me? That pretty much everyone whose ever lived has gone through this. Anyone whose ever been with more than one person has felt this way. Practically 1/3 of all music ever written is about this feeling. I mean, look at all these people commenting going through the same thing. Maybe that's not comforting to you, but it makes me feel a little less pathetic.
I recently met someone who has a lot of similar interests as me and we've been casually dating. It's by no means serious right now, might never be, but it does help with the pain. I'm starting to feel the way my ex used to make me feel again and that feels pretty good, even if this doesn't become anything serious. Maybe you should try dating. Even if no other girl compares, it still feels good to have someone close to you again.