I'm going to agree to still be with my cheating girlfriend and still get mad at her
Why would you agree to that?
I'm going to get pleasure from seeing her suffer.
Staying with your girlfriend specifically to make her suffer is an awful thing to do. She shouldn't have cheated, but you aren't making the situation better by not letting her seek forgiveness. It sounds like you both need to talk to an expert.
I don't want to listen to an expert, they will just tell me to leave her. I don't want to do that, I am just a sick person who can't be helped.
Yes it seems you have an issue, but therapists and medication can fix it. These days there are all kinds of new technology and medical practices that help everyone. Enjoying someone's suffering is unhealthy to say the least, plus she is hurting you too.