Yes, it's very upsetting to be rejected by a guy you pursued and fell for. But, it happens. And breakups are never any fun. But, guys fall into two categories:

1. Guys who are interested in you (you don't have to chase after them or smother them)
2. Guys who are not interested in you (they are too cowardly to tell you they lost interest, so they will make you do the work of breaking up with them, by mistreating you)

This has happened to us all - who date other people.

Red flags: if you have to chase after the guy and smother him or nag him to spend time with you and prioritize you in his life.
Green flags: He pursues you. Prioritizes you. You never have to guess what he feels about you.