I think a good place to start would be to stop referring to your feelings as pathetic. You're treating your feelings as if they're just a pesky annoyance rather than something that matters.
He treated you as if your feelings mattered, and that's what he was doing for you that you can't do for yourself. Your friends can't make you feel like your feelings matter, your hobbies can't make you feel like your feelings matter, your social life can't make you feel that your feelings matter... only he could do that.
But you can do it for yourself. Your feelings aren't pathetic. You keep thinking of him because you have feelings for him, and that's totally valid, and something all healthy people do: have feelings for someone. So, nothing pathetic there.
Stupidly I thought it was the girls fault
You thought it was the girls' fault because you were told that that was the case by someone you trusted, so that wasn't stupid, that was the action of a standard, socially driven human: you believed something you were told by someone you were close to. So, nothing stupid here.
Then, you were duped, by someone who did a wrong thing to you. So, instead of calling yourself/your feelings pathetic and stupid, perhaps you could adopt a more sympathetic attitude? If a friend told you she'd been lied to and left, would you tell her she was pathetic to feel sad and stupid to have believed him in the first place? Can you see how mean that is?
Start being nicer to yourself. You've had a rough time; be on your own side. What will make you feel better? Chocolate and Netflix? Distract yourself by starting a course? Go mountain climbing? Knit? Do nice things for yourself. Say nice things internally about yourself. Feel nice things towards yourself. It won't be long before you start to feel like being treated so well internally is going to be tough competition for any other person to beat... and that's the self confidence you need to be alone and to feel proud that you do so well without abusive company.