Just to start you off I'm 23 and I've been dating a girl that is almost two years older than me. We went to college together and live about 5 minutes away from each other. This relationship has been one month shy of 5 years. The relationship was kind of rocky in the beginning because of trust issues that she lived with on a daily basis between her mom and dad. But as time went on we worked a lot of things out.

My issue is she can't trust me anymore because of an incident that happened between me and a HS friend/ex girlfriend that I dated for about two weeks. Problem is that we told each other we were going to be friends and that was it. That statement still holds true but I've been going to her about my problems with my current girlfriend to vent out.

The problem is that my hs friend called me up randomly about two years ago to go on a joy ride with her bf's car. Me being a car nut was like sure. That day I took my SLR camera out with me to take pictures of the car. At that moment she took pics of me and I took pictures of her. There were no feelings between us but my girlfriend thinks otherwise. I never told my girlfriend about this incident because it would have made her blow up about it so I kept it from her. She found out when she found the pictures online. And we've been fighting almost every other month about it. It comes out of the blue when I least expect it and she just becomes depressed. I didn't find this to be such a big problem because I don't have any feelings for this girl and I never cheated on my girlfriend with her.

There's more to it but that's the core of my problem. My relationship is falling apart and I can't fix it because she tells me she hates me and refuses to forgive me for it. She's been sending me hate texts saying she's going to go eff some other guy to get me off her mind. I tried to look at things her way and I apologized to her but she thinks I'm still lying to her. So yeah I've been a sucker and I tried begging her back, I keep trying to call her she picks up and hangs up on me. I go to her house and she ignores me. Deep inside I'm really hurt and I wish I could take back that day. What should I do now?

I'm sorry you are having such a difficult time. I'll give you some advice, from someone who is about twice your age.
I would guess that your girlfriend became more upset that you were "sneaking around" with the hs friend than anything else. Since you kept it from her, she assumes you are being dishonest, even if your intentions were not. The best thing would have been to ask her if she would like to come along, if you felt you HAD to see this car. This would have shown that you were sensitive to her feelings. It will no doubt upset her that you are discusing her with this friend. I think it would be best if you only "hang out" with the other girl when you girlfriend is with you. It's easy to understand that since you have lied about things in the past, it is difficult to trust. Don't keep things from her!
On the other hand you say she does have some issues of her own. She is still young and not mature enough to know that revenge isn't productive. It's also possible that she knows she is in control when you beg and plead and she probably enjoys that since she feels out of control a lot. I would leave her alone for a few days, maybe telling her that you are giving her time to think about what she wants. Let her calm down. In a few days, have a mature conversation with her about where the relationship should go. Decide if it can become healthy or if it would be best that you both move on. Just some thoughts from someone who has been happy in an 18 year marriage. Good luck!