It still gets to me sometimes and it's been 9 years!!! I don't have kids, either, it's just those those flamin' "what if?"s and "was it me?"s. At least, these days, I know what's going on.
Bad relationships leave more of an aftermath than they should. When you've spent years of your life believing you can make it okay ... that you love each other really ... that if only you tried harder, tried less hard, tried believing in him or believing in yourself ... that's a whole lot of trying. The harder we tried, the more invested we became in getting the happy outcome. It never was going to be a happy one, but we weren't able to admit that, were we? So we kept on trying.
Then, whoosh, it's all over. All that effort, all the changes we made to ourselves, all that hope in the face of the facts (!) not only gone but wasted. It's hardly surprising our poor, overworked brains still hang onto the idea that maybe it could work - or could have worked. Old habits are hard to break, especially when you put so much into developing those habits in the first place.
You say you want him back and you want you back. That makes me think you're suffering from exactly the same problem! Who is the "you" you want back? Did you change yourself, twist your self out of shape a bit, for him? Maybe you invented a "you" to go with the "good" marriage you made up in your head? I know I did. I'd go as far as to say I actually forgot who I am - the wife I invented, and the wife he said I was (not the same wife, by any means!) took up all my thinking and there was no room left for me to be myself, iyswim.
I think you're missing the dream - the man you thought you were marrying and the marriage you expected. Not the grinding, tiring, confusing reality.
Lots of things have helped me along since then, and one of the most powerful is posting to Mumsnet. Discussing my painful relationship, with other women who've experienced similar, gives me a reality check. I didn't marry the sweet, thoughtful, sexy bloke I fell in love with, he was a fake. I married a self-centred liar who hates women. When my brain asks, yet again, "What could I have done differently?" - there is only one correct answer: I could have recognised the signs instead of ignoring them. I could have chosen not to delude myself.
I imagine you're thinking it's best for you not to keep thinking about it. But maybe, in fact, you need to think about it more - write on here about things that went wrong, and start building up the true picture of the relationship you miss so much. You may well find it's not worth missing after all ...
Good luck :)