I'm not going to show my girlfriend any more attention.
Why would you stop showing your girlfriend affection?
I have decided not to care about her anymore.
I think it is wrong to just stop caring for someone without good reason. You should be affectionate toward your partner when you are in a relationship. You should try and talk to each other about how you feel and your needs.
She needs to learn a lesson. I'll do what I want.
That is a very inappropriate thing to say about another person. If you are that unhappy then break up with her.