Okay, so about 5 or 6 years ago I put my current girlfriends sister in a headlock. At the time I and he sister were young, and I didn't mean to hurt her in any way, shape or form. When I put her in said headlock I made sure that she could breath and that I wasn't choking her. Her sister was apparently traumatized by this and cried to her mom how I attacked her. It's been years and my current girlfriends mom still will not give me a second chance to prove I am not a bad person and that I did not mean any harm to the sister. Her sister has come to realize I did not attack her and that we were too young for it to matter now, her mom said she likes me but she isn't going to give me a chance, but only accept me because I'm with her daughter. I have apologized countless times in the past 5 or 6 years and each time I see her mom or her sister I feel more and more like crap for something I did years ago. I know that this probably makes no sense or it seems as though I am the bad guy here, but I wanna know if anyone has any ideas as to how I can either stop feeling like crap about this or how I can get her mother to give me a second chance...

I don't think it seems as though you're the bad guy. The only problem I see is that you're dwelling on what happened in the past. It should be clear by now that you're sorry about what happened. Everyone makes mistakes and misunderstands each other. Her mother accepts you, which should be enough. Of course, I could be misunderstanding as well. I believe you should move on from what happened, let it go. Don't act like it never happened, but apologizing more isn't going to change anything either.

It doesn't help to dwell on the past. Whether or not her mom accepts by now shouldn't matter. If she's been holding a grudge for this long, that's her problem.
(Sorry if it's a bit vague... >.<)

I hurt someone once too. but she needed to hear the truth of what she was doing wrong in her life and I told her. she's never forgiven me for that and I've apologized so often I began to realize and question myself: " why am I still apologizing for something she can't get over? I'm over it, I've apologized more than needed." don't beat yourself up over it. and honestly, forget about it. she's the one still grinding her teeth in her sleep while you can sleep peacefully my friend. it's a hard rut sometimes to get out of. but it needs done.