Kudos for having the extra lid for your girl.
The problem isn't your girlfriend.
Sportbikes are different than regular motorcycles. Your passenger is stretched around you in a prone position, usually with insufficient upper body strength to support herself for extended periods of time. Your helmets are naturally within close proximity to each other, making it difficult for the passenger to see around the driver's fat head. She can't see what's going on and can't anticipate your every move.
You're the one accelerating/braking/driving so erratically that she is helpless to stop it. IF you can't slow your bike down without her helmet banging yours, you're going too fast. If you can't accelerate without her helmet banging yours, you're accelerating too hard/shifting too slow. Maybe you can safely manage the speed, that's not the point. Your girl may be getting fatigued or just be unable to support herself when you stop so quick, it's like doing a full pushup unexpectedly. Have a little compassion.
IF you're getting upset, she's likely MORE irritated: She's trying to be cool and share your interest, but you're causing the problems and getting mad at her for it.
Cut her some slack and quit ruining the girl for the next cool guy with a bike!