plus if the girl turns herself in how would that help me? I'm being charged with assault not her.
As for the girl turning herself in, if she is the one that actually committed the theft she may be worth more to the prosecutors in terms of conviction than your assault charge. Just like a getaway driver turning in the bank robber. The bank robber is who they really want, and in most cases they will let the getaway driver off with a slap on the wrist if they help get the bank robber convicted.
I can't reiterate enough though that I am not a lawyer, not familiar with your case, and this by no means legal advice by any stretch. The only honest, been there done that, advice I can give you at this point is that in situations like this you need to step back and assess the direction you are headed. If you are truly remorseful, this was a slip in judgment, and you are honestly wanting to get your life headed in the right direction, you need to remove yourself from your current circles. Snitches get stitches is all fine and great when all you aspire to is life on the streets or in jail, but if you want to change for the better you have to look out for yourself first. That means taking actions that can be difficult at times, like completely dropping the circle of people you hangout with (they aren't your friends if they are robbing stores when you are with them) or moving out of the area.