In going through a breakup right now and dealing with the knowledge that my 2.5 year relationship was extremely simple for my ex to shrug off and find someone new after 2 months, I re-read the sidebar links. I followed links within those links to a post entitled "War Brides." It explains why women are so good at moving on. I think I can sum it up, better:

She gets over you quick because you are disposable, and she has to be ready to say goodbye in case you are disposed of.

Back in the day, it could have been because of war or hunting accidents, or what have you. But now we're here. Since men are disposable, women are the ones that have to put up with the consequences of our disposal, so the quick moving on is a defense mechanism. Since women are not disposable, we never had to acquire this ability.

Might have been nice.

EDIT: To those giving the "man up" response...yeah, I agree. I hooked up with someone sooner than she did, and I even got picked up at a bar another time. Both very quickly after our official end, which was in fact mutual. The kicker with all this is that we were not able to move out of the apartment we share. That's happening this month. There are financial and logistic reasons for that. What it has amounted to is that now that she has a "someone," I'm exposed to things like knowing when she doesn't come back for the night, and watching her primp and preen for hours before she goes to see him. And of course, now since she has him, she is completely robotic to me in ways she wasn't in the old post-breakup phase. It's Draconian-level psychological torture. I don't care how much you think you have your shit together, if you have this shit literally in your face, it fucks with you. And yeah, I've been going to the gym a lot more and trying overall to be better. This is simply a tougher situation. All I want to do is get out of here and forget she exists, like what normal people do.