When I have sex with my girlfriend and I want to come I fantasize about others. I am upset about this, because I only want to have sex with her. What can I do to stop this?

To be able to climax people go through the solophase. In this phase you shut out your lover and you concentrate on your own feelings, thoughts or fantasies in order to come. Unexpected images may pop up, like dreams in the night. These fantasies about other girls are only disturbing if you interpret them as wishes. But fantasizing about other girls doesn’t mean you want to make love to them in real life. So accept your fantasies for what they are: fantasies. If you suppress them you disturb the solophase and it might become harder to climax. That doesn’t make sex more fun, for neither one of you.