I'm afraid my girlfriend is gonna cheat

Okay, so I've been with my girlfriend for about three months and I'm completely in love with her and trust her completely. But I'm always worried that she'll leave me or cheat on me like my first girlfriend I fell in love with did. I don't think she...

would and she says that she wouldn't and I believe her, but there is this part of me that doesn't. Why do I keep thinking this?

Because it happened to you already, try trusting her a little more, I know you said that you do but if you fully trusted her then you would not be thinking it.
Try to pretend that other girl didnt do that
she no longer exists, because you cant let it follow you around
or your relationships will fizzle out because she will eventually get sick of not being trusted for what another girl did.

If you're afraid that she's going to cheat on you or leave you, then you don't trust her.
You need to realize that not all girls are the same, and she isn't your ex that cheated.
I know how hard it is to trust someone after that, but you're gonna have to learn how to before you lose her.
Relationships can't last without trust. You think that because you believed the first girl when she said she wouldn't hurt you and she did anyway..
Just forget about your past relationship and focus on this girl... she's given you no reason to believe that she would cheat or leave you.