About my girlfriend cheating?

[Maverick]

I'd say, leave her now, while you are still young and can find someone else. She may apologize, but she'll do it again.
"Cheating" is common when a relationship has continued for a long time. not when you are both young.
Sooner or later, ALL men "cheat". It is how we are wired. That some women don't understand this is clear.
Hogamus higamus, men are ploygamous.
Higamus hogamus, women monogamous"
(William James Psychologist and Philosopher.)
The bit about women is no longer true, by the way.
Yes, it hurts, but think about the reality and come to terms with life.