Why do we push away the people we want the most?

I know this is a faq, but I recently let go of a treasured boyfriend, because our relationship wasn't healthy. But now I still like him a smidge, and seeing him with other girls hurts. Why did I do this, when I knew there was a big chance we could've worked through that stuff together and become stronger as a couple? He wanted to stay together, but now he doesn't want anything to do with me. What have I done? What can I do now?