Got cheated on

Ex boyfriend cheated on me with this girl at a party. She also knew that we were together still at the time and that we were together for 5 years. He broke up with me but didn’t tell me because he cheated, instead he just told me excuses of why he wanted to leave and now he’s with her so he basically left me for her. It hasn’t even been two weeks, ( it will be two weeks tmm) and they are already going out. This is crazy, idk how he could just leave me for her after everything that we have gone through together.