How can I stop thinking about him

So I’m currently in a relationship with my bf. We’ve been together for about 8 months now. Before we got together I had to cut off a few people that I was taking to. One of them I had a lot of feelings for and we had sex a few times. Once I got in a relationship he would still try to talk to me but obviously I couldn’t. Anyways I had to block him on everything because it was getting out of hand. It’s been months since I blocked him. But although I did that sometimes I still think about him which is horrible. Which I don’t get because I’m really happy with my bf right now and I love him. But the boy I used to talk to pops in my dreams sometimes and it’s annoying because he needs to stay in the past. And the worst part is it’s not like we were ever a couple or anything. How can I stop this? Any advice?