Is it good to ask for my friends advice, since I think my gf is going to leave me for a guy she's incredibly defensive about
They could help. Why do you think your girlfriend is going to leave you?
I know she will leave me, it's inevitable.
Even though you think it is inevitable, it is good that you are talking to your friends about it. Maybe they will give you some good advice to help your relationship.
I don't want to fix it. Honestly I just want to fuck a bunch of girls.
Cheating on your girlfriend at this point is the last thing you want to do. If you really don't want to be with her anymore, then you need to break it off with her first. Cheating on her will only add to the tension you're feeling with her.