I've been cheating on my girlfriend for a while and I'm the only one who knows
Are you going to tell your girlfriend that you're cheating on her?
I don't plan to tell her about it.
Cheating on your girlfriend is an awful thing to do. When people are in a relationship they are expected to be faithful to their partner, and that means not cheating. You should stop cheating on your girlfriend right away.
I will stop it once I'm done with the good sex that I like.
That is really terrible behavior. You are being dishonest and ruining the relationship with your current girlfriend, and her ability to trust other partners in the future. You should break up with her right away and be honest about what you've been doing.