Sex is strange for everybody. Many people lie, both to themselves and to others, about this, but it is true. Everyone has strange fantasies of one kind or another. Of course, not all of them are violent and sadistic like yours. Some of them are masochistic instead. For example, someone may become aroused by imagining being whipped or made to lick someone's butt. If you had that kind of fantasy instead of the kind you have, would you be happier.
I think you should stop worrying and just enjoy sex. But, and this is a big but, if you ever think you would really hurt your girlfriend, then you should get help right away. Any kind of fantasy is fine, but not every kind of reality is fine. As long as you know this, have a good time and enjoy your orgasms.