I'm telling you now, this will happen again. They may say they love you, but they just love the idea of you and having someone there to avoid feeling lonely. He's clearly not mature enough to be loyal. Even though he might not love the other girl, he still has attachments that he should have cut off. Instead, he should have started anew with you, but he didn't.
A man is like a job—once you quit, you never go back. Start a new chapter in your life and focus on self-improvement. Maybe in the future, when you both have grown up, you can reconnect.