Why do I still yearn for a past love?

A year and a half ago, I split from a love cause we were both too far apart from one another. We also had our own "challenges" to overcome.

I am getting my life back together, got myself in a good relationship, but lately, thoughts and feelings of this past love has emerged, stronger than ever.

It hurts. Not sure what it is cause I never felt this way before. She was/is something special, however, there are reasons why I should not go back to her as well as reasons why we belong.

Much confuse.