From the movie Chasing Amy, it means when you dump someone who was perfect for you for a reason that you eventually realize was not worth ending the relationship for. However, by this time it's already too late. The other person no longer feels the same way about you, but you will spend the rest of your right regretting this decision that you can never change.
G1 - I'm falling hard for this guy, but I don't think I can stay with him.
G2 - Why not?
G1 - He tries to act like he isn't, but he's definitely still in love with one of his ex-girlfriends. I'm pretty sure he's Chasing Amy.