Why Am I hung Up on a guy?

So, my past 3 relationships I haven’t really cared much for the guys. And they lasted for about a year each. I was never heartbroken when it ended. But I started seeing a guy for about 3 weeks, we cuddled and kissed all the time. I was always with him when I wasn’t working.

We had sex and few hours later he broke things off because he didn’t feel ready for anything serious anymore. But for some reason I keep looking at my phone just waiting for him to text me and change his mind. I even cried for 2 days about it, so I don’t understand why I am so hung up on a guy I only saw for 3 weeks, and yet I didn’t give a crap when my long term spouse broke up with me ??