Hi there,
I can't trust my girlfriend. Period.

We've been together on and off for more than 5 years, when I was 17 and she was 15. She's my high school sweetheart.
And she cheated on me little over two months ago. Well, before she cheated on me, we were having a rough time.
We were constantly arguing/fighting over same issues, rough time, and she found a 'friend' who was treating her like a princess. Ok that's fine, I'm cool with that, but the night of our 5th anniversary, she got drunk and told me that she no longer know which one she likes more.

Ha,

And then, we broke up on the Valentine's day, which hurt me more, but then like 3 hours after we broke up, she went for a dinner with that guy. I found out because she told me so.

Ok that's all good now, we are back together, but, we had been constantly fighting and arguing again because she didn't want to cut the connection with that guy, even after we are together and they broke up. So after like 13 nights of suffer, I called it off. I told her that I can date someone who cheated on me previously, but I can't date someone who is in a constant relationship with the person she cheated on.
So, she said, OK fine, I'll ignore all his calls, texts, etc. So we were good again.

But the main problem is that,, I can't trust her anymore.
She used to call me every night before she goes to bed, and I used to hang up when I think she's asleep.
But she doesn't do that no more which is cool but when we are not on the phone at nights, I just can't help myself thinking she MIGHT be with that guy. OK, she meant so much to me, and I went through various psychological breakdowns and symptoms during the 2 months we broke up.

My question is that, is it normal for a guy who was cheated on not to be able to trust that person?
And, if I can't seem to move on and fully trust her, what should I do?

Just ignore whatever happened and unconditionally trust her?

She is going to a party this Friday night, and I can't,
She'll get drunk like hell and I just have no idea what that would feel like to me...

Please help