Broke up 4 weeks ago and already has new girl

Does anyone have any advice on how to deal with a breakup. Me and my ex were together for not a huge amount of time but to me it was serious (we were together for 6 months). He was the one I lost my virginity too and also first guy I had ever seriously loved. 3 days after we ended he was already hanging out with a new girl (who now is his new “thing”). I’m finding it incredibly hard to deal with seeming I now feel like I have no support and no one to turn back on. I changed my whole life for him eg. Left all my friends at the time to become friends with his friends, I gave him every single thing I had and he still left me. He broke my heart into a million pieces.