I don't know if it's just me, if I don't get the signals on time but I've always been dumped. I've never broken up with any of my girlfriends, but they've always left me.
Yes, there's probably a recurring reason why. Are you emotionally needy? Or a workaholic? Girls hate those sorts of things. Are you still friends with any of your exes? Maybe you should ask her for an honest assessment if so.