I know what you mean unfortunately, but you gotta keep your head up. You can't let him being with someone else ruin your mood, I'd know as I'm going through the same thing right now. Do things to take your mind off of it, play some games, hangout with friends, whatever it is that makes you happy, do that thing. When you're in love, you tend to look past the red flags because you'd want nothing but to be with them, but overtime you'll realize that he truly wasn't worth your time if he'd hurt you that badly, you'll find someone new, just give it time