I'm going to tell my girlfriend we are not very compatible sex wise and ask if we can have an open relationship
What kind of difficulty are you having with your girlfriend when it comes to sex?
She doesnt want to talk about it and be open with me.
It is good to be honest and open, you might be better off breaking up. She might get really offended if you ask to have sex with someone else.
I'm guessing she's going to be hurt and I will be okay.
Being honest sometimes means hurting a person you care about. It's better to have a difficult discussion though if you really want to save the relationship. Try to respect her feelings while you're having this discussion with her.