I feel like I'm holding a grudge.
Why are you holding a grudge?
My girlfriend was cheating with my friend. I'm going to break up with her.
It's normal to hold a grudge against someone who has hurt you. You should try and let the grudge go once you have broken up with her so it doesn't weigh on you for too long.
I'm never let it go. I'm going to fuck his family and friends and make him suffer for hurting me.
I know you're upset but I think that's going a bit too far. Just try and forget about her entirely. A grudge can just end up eating you up inside.