When is he going to be gone from my head and thoughts?

I am not obsessing over him constantly, but thoughts of my ex do return probably a couple of times per day. I don't dwell on the thoughts either, I let them come and go and they leave rather quickly, but it's annoying af that I can't seem to get him off my mind.

Is it because I am single and alone now and he was the last person I was serious with? I mean, it's not like I haven't met or been with any other guys since him. I dated one casually for 2 months, and I've slept with a few men since. And right now, I am playing the field a little bit so to speak. But thoughts of him won't leave me alone.