Have you ever had a real relationship before?
The fact that you worry so much, is usually a sign of not having enough trust. Which is something you have to build over time. I was jealous in the beginning too, it's also a bit of self worth, accepting she wants to be with you. If girls don't wanna be with you, they wouldn't and ran off already.
Don't be suspicious of her cheating, she is not (high chance). If she is, and you have proof or have a good basis to confront her about it. Then say it and break up.
But until that time, try and find peace. Accept that she is doing her own thing and not clinging onto you like a small child. Love her like you would and stop bringing yourself down, just because she is going out.
Also, it's always good to be vocal about your feelings. To each other, talk about it. Don't make restrictions, but make her see what you see. Then she will too.
You're probably young or inexperienced, or just a first timer.
Hoes don't fall for boys who game a lot and have not much to offer material wise.
Be cool and stop making yourself crazy.
TLDR: Accept and stop being a pussy :)