You just have to keep in mind everyone is human, and makes mistakes. When people are in certain frames of mind they'll say things they don't mean... it's important not to hold this against them, because doing so is only a burden to you.
Yeah, Derrek is completely right...
As for tips on how to pull it off - remember that you, too, will find yourself exactly in his place some day (maybe something comes out wrong and he takes it the wrong way) - how would you like to be treated when that happens? Sometimes, this kind of thinking gives me the perspective I need to be able to release whatever it is I'm holding on to.
Also, have you thought about why it hurt so badly? I've found that when someone hurts me unintentionally, it is very often because of pain I experienced in my past - and usually inflicted by someone else. By really thinking it through, you may be able to find the root and weed it out. By way of illustration, say a little child runs up to you in joy to give you a big hug... it's a sweet moment and brings a smile to your face. Now imagine that you broke your toe a while back, but it never healed, and is very very sensitive. Now, when you see that child running toward you, you cringe and put up some defenses, maybe putting your hands out so they can't get near your toe - if they happen to step in the wrong place, they could really hurt you. Sometimes it's good to find the real source of pain so that you can mend it and bring healing.
Oh, and as for forgiveness, I find that if I focus for a while on really loving that person and building up new good memories, eventually my grip relaxes and I'm able to move on. Sometimes, when we aren't able to release bad feelings, it can work to replace them.