Why am I so jealous?

I dated my best friend since I was 5 for a couple years recently. I loved him more than anything and if we weren’t so young (20) I would have married him at the time. But we had a lot of issues and he was very immature. So I ended things with him 6 months ago. Like a month after we broke up I met a new guy who I love very much. I am currently dating this new guy and I feel nothing for my ex, but I just found out he (my ex) has a new girlfriend. And I am with someone else and I have been for 4 months so I can’t be mad. But I’m really jealous and I don’t know why. I broke up with him and I would never tell him I’m jealous because it’s not fair. Is this Normal and okay? Is it fair to my current boyfriend that I am jealous? I know I don’t have real feeling for my ex.