How could I think to feel better?

This is a stupid question but I don't know how to arrange my mind or how I could think to feel better.

I ever had a boyfriend and we were together for 8 years before he broke up with me. Times past for 4 years until I started to moved on. Now I'm with my new boyfriend for 3 years but why I'm still thinking about my exboyfriend. 😞 I didn't want to go back to the same relationship and my new bf was very good to me. But in many things my new bf can't understand me. I can't talk with him cause he don't have knowledge of that e.g. About my hobbies or things that I like to learn about.

And there are many things that even my friends, my closed friends or people around me can't talk to or discuss about it. For example, I like learning about plants - taxonomy and all my friends can't talk about this topic or discuss with me. So every time I have to do something about this topic I was feeling so lonely and thinking of him.

I'm so tired of myself. How can I do?