What if I'm not sure I love my girlfriend anymore?
That's not good. How long have you felt this way?
I've had this feeling for a long time now.
People fall out of love with each other. If you've truly re-examined your relationship with your girlfriend, then it's better to be honest with her and end the relationship, then to lead her on.
I'm not sure if I want to end the relationship, but I want to make her feel like shit first.
You don't have to stay in the relationship but you shouldn't make her feel like shit. It is better for both of you to just end it nicely so that you won't have bad memories of the relationship.