How physically attractive is she? How outgoing and charismatic is she? Does she have an adventurous approach to sex?
If she's not that attractive, or not that charismatic, or not sexually adventurous, what are you doing with her instead of getting yourself a girlfriend that is?
If she is all of those, you can expect guys to hit on her and make a move on her all the time.
And given the context of you being hours away and not seeing her that often, it would be understandable from her point of view if she slept with another.
The only things in your favour would be that most students are poor to mediocre at Game. And the integrity of your girlfriend.
You CAN'T have it both ways: a top level girlfriend for you, that somehow becomes invisible when you're not there.
The best you can do with a desirable girlfriend in this context is to use reverse psychology.
Tell her that you love her. Very much. And that you love her so much that want what's best for her from her point of view. So that if she ever comes across a guy to whom she's attracted, a part of you would want her to try him, whilst a part of you would hate her doing this, but the part that wants the best for her would win out. So that you hope she'll have a great social life at her uni, and you realise that she may fall into the arms of another. And that if that happens you'd like her to tell you about it, before getting into bed with you again. Although you'd understand if she kept it secret.
And that if all this happens, you'd want to sit down with her and take a rain check and decide what would be the best way forward for each of you from there.
If you've got your head screwed on properly, you will have an active social life at your uni and if you come across someone you're attracted to, you'll make your move. And if that move leads to sex you'll let your remote girlfriend know about it.
You should be aiming to be an attractive man. You should be aiming to become (if you're not already) the joint best boyfriend in the world. And as such a man, you'll have options.
When it comes to trust, you should never fully trust another person. Because we, as human beings, can change, and we can be very good at lying and keeping secrets.
So that it's all about balance of probabilities. It may be probable that she's been faithful. It's never definite. That's OK. It's not the end of the world if she has sex with another and she keeps it secret and you never find out. It's also not the end of the world if she cheats and you find out. All it is is the end of one chapter in your life and the start of another.
There are ways you can behave to reduce the chances of your being cheated on. The reverse psychology is one of them. Others include you being the joint best boyfriend in the world. Others include you developing your empathy and ability to read people and you applying that to whom you select to be your girlfriend.