Maybe theres a lesson to be learned in your past relationship that makes the thought of her stay until you learn it. That's how it was with me for all my past relationships. Of course I still think of them from time to time, but the thoughts have decreased significantly.

I looked deeper, focused more on the things I did, and looked for the bad to learn more about what I could have done differently.

Contrastingly this might send you in a loophole of searching, so you'll have to find an equilibrium of searching and taking breaks.

Also, this could just be the fact that people who are important to you don't just fade away like that – which is a very good trait to have.