How do I forget about the past and live happy with my gf? Around

Sounds like you know what you have to do. a emotional affair is still a affair, and she went on a date with another guy. that's fucked up, it'l be hard to distance yourself but if your not attracted to her there is no point in sticking around

>>16892522
I have been in your position, even if I'm a girl.

Do not tell anyone. They'll clearly get mad at her and run things. There's enough stress.
Take time off her. Make it clear that she has no right to even think about someone else during those days.
Take some days for yourself. Think about all you've been through and try to forgive her. Do not talk to her till you're pretty much over it and ready to forgive her. If you can't, leave her.
If you choose to be back to her, do not talk about it anymore. Do not mention it in fights. Never.
I asked my boyfriend to tell me his passwords and let me check his phone for a while because I felt insecure. I asked him to check 2 or 3 times in 4 years. Obviously I gave him mine.
We slowly got trusting again. Took me a year to feel attracted to him in the same way, and to respect him the way I used to before.
Good luck OP.