I cheated on my gf with our cousin
I'm horrified to hear this. When did you first have the urge to cheat on her?
I felt like cheating during the vacation because my gf told me not to.
You should have probably listened to her! It is wrong to cheat on your significant other, especially with a person who is a relative.
I might make her pregnant so she will never cheat on me.
I do not think it's fair with your girlfriend. You have already cheated on her and that is already problematic on itself, but what is worse is that you're now going to threaten her to not leave you with a pregnancy. I think people will see you as a toxic boyfriend and I wish you treat your girlfriend with respect instead of cheating on her and locking her down as she deserves better.