I'm going to break up with my girlfriend only for the summer
What is the reasoning for this idea? Why would you want to break up only during the summer?
I'm going to date other girls and I intend to date them only for the summer
That is a terrible and hurtful thing for you to do. If you break up with your girlfriend to be with other people then expect her to not take you back. You shouldn't hurt or disrespect her that way.
I don't know why she would take me back. I'll go ahead and break up with her.
It is not normal for people to break up just for a summer. Usually, when you break up you do not expect to get that person back.