You are only human, as are we all.
It’s a mind thing - you want the most what you can’t have. I understand you - you love your girlfriend, the sex is great, and if she was someone you just met you would want to fuck her all day. But when you really can fuck her anytime, it’s not that great anymore.
It’s the fucking human brain, and there is no way to deal with it except accept it.
What’s funny is that any other girl could probably get you horny with just eye contact, but your perfect girlfriend has to work on it.
But trust me - if you were to break up with her, you would again be attracted to her like crazy simply because she is not your girlfriend.
It’s annoying, I know, but that’s the way it is. Fucked up.