He moved on

So this week my ex boyfriend ( but he was my boyfriend at the time) had been ignoring me. And he usually does this often when I upset him but this time he actually found a whole other girlfriend and they started dating the same day I was over his house chilling and having sex with him.. I have no idea how to feel. I just feel hella used and like I can't seem to move on or forget about him. And break up advice ?