What should you do if you like someone but that person has a girlfriend?

I'm sorry to hear if you are going through this, I am as well going through the same situation. I guess we girls just have to move on with our lives, there is nothing we can do to make them break up, or make him like you. The guy I like has a girlfriend that I have always envied. She's blond, she's skinny, she's pretty, and she is funny [not so much smart], you can say she is your average "stick my finger down my throat, sleeping with multiple men, gets everything she wants, daddy's girl" kind of slut. Excuse my profane language but I am an average bad public middle school-er with a sinful mind. Hopefully moving helps you and anyone else.