I'm going to tell my girlfriend I won't go see her play and don't want her to do it
It's her life and decisions. What if she broke up with you because of that?
Her life will suck more than it does now, she'll be so miserable without me
She will move on without you if you tell her that. Nobody likes when their partner tries to control their passions.
I will ensure she won't break up with me
I feel like you're being a little too controlling of what your girlfriend can and can't do, are you really okay with that? It's not appropriate because she is a human, not a pet bird. How about instead just talking to her why you are concerned about her play?