Just my personal opinion here.
I think it is normal to still have feelings for your ex even when you are in a happy relationship/marriage now. You probably have spent quite some time together with him and had happy moments. However, for some reason, things did not work out. Often, with time, we tend to forget the not-so-good times and only recall happy memories. Just cherish those memories and remind yourself how happy you currently are with your partner.
For me, I feel that there is no need to try to actively forget the other person. But not acting on any feelings you think you may still be having is key.
Take a moment to think of all the things you are thankful for in your current relationship. Even the littlest things count. This may help you stay grounded. :)