Is there a disorder for thinking too much?

Im constantly thinking about negative things specifically my girlfriend. Like this is my first girlfriend I'm 21 an I didn't know Im so unstable and insecure about myself. Like im always thinking about her and her cheating on me and doing all other stuff and not actually likin me. I don't like always thinking about stuff and I don't like it and I seriously can't stop it im always thinking and I can't help what should I do