I'm not in a relationship, in fact I've never had a girlfriend (been saving that for after college). but I do know this. There are several key things that there need to be for a relationship to exist, and one of those key things is honesty. Without Honesty there is no relationship, just lies. I may not be in your possision or really in a good enough one to give you the best advice, but I just wanted to say that I agree with hintongirl23. Not telling your partner is only going to hurt you and them WHEN they find out the truth, and trust me, the truth always shines through, no matter how much you try to throw on top of it.
Keep going man, thats all I really can say. God loves you, God forgives as long as you truly want to be forgiven and are truly sorry for what you did. If your partner has the same unconditional love that God shows us everyday, the unconditional love that we should have for our families and loved ones, then she will forgive you. You need to sit down with her, look her in the eyes, let her know that you truly love her, and would do anything for her, while still looking her in the eyes apologize for your mistake. After apologizing tell her what you did and then maybe apologize again.

Bringing in flowers in the very beginning never hurts either. I'll be prayin for yah man, just hang in there, things get better once you give it up. Things WILL in deed get better, no matter how bad they may seem to get.