Married, but thinking about ex.. HELP!

I don't understand and I don't know how to make it stop. Here's the thing.. I can't stop thinking about my ex and I'm happily married. Here's the back story: I met my ex in our late teens. We were so in love, we really took our time in our relationship and grew so strong together. We lasted 5 years.. People started pressuring him to propose to me. He knew I wanted to get married and have kids more than anything. Towards the end of our relationship we had a lot of ups and downs but it all boiled down to him deciding he never wanted to get married or have kids. He decided it wasn't fair to me to keep dragging me along, so we broke up. It was hard. I spent a year alone, trying to get over him and learn what I wanted in life. Then I met my husband. We fell in love hard and fast. My ex contacted me at this time to get back with me, I told him I moved on, he accepted that. (he's now married with a child, and we have zero ties, not even social media but that's irrelevant) My husband and I got married, have been together 6 years now.. Married close to 4. He's an amazing man, he's helped me achieve so many dreams. I love him with everything I have but for some reason I can't stop thinking about my ex. Like sometimes he crosses my mind daily. It's even to the point where I've cried. I know I don't want to be with him, I know everything happened for a reason, I know my husband is the one for me, but why can't I stop thinking about my ex. I don't understand, I just want it to stop!