Ok. I had a more complete answer than this, but my phone sucks so this one will be a little shorter. Go be somewhere alone (preferably in bed, in the dark) and go through all of you're worst memories and think of what you could have done differently. That's my short answer. Though be warned, it could screw you up in other ways (make you depressed). That's what I do if I really need to cry but haven't been able to.