Advice Please! What To Do?!

I have been divorced for a little over a year now and I’m super happy! I’ve gained weight, moved into my own place, going back to school. . . Just enjoying my life. I even have a new man and he PROPOSED! I’m soooo excited and cannot wait to marry him. The only thing is. . . I haven’t told my ex and idk how. Now, I don’t have any feelings for my ex at all. It was a very toxic relationship. There were a lot of lies and control. I didn’t even love him we only got married for our child. The thing is. . . It’s like he and his family always had some type of like control over me and made me very fearful of them. They never hurt me physically, but they definitely broke my spirit. They always made me feel like they knew better, were better and that my child was better off with them and not me. Even when we got divorced my ex pushed for full custody so much that my attorney was like girl is there something you’re not telling me?!? I have ALWAYS had my child. My child and I only lived with her father for 6 months, and that was how long we were married for. After our divorce I even moved to a town closer to where he resides and offered him 50/50 custody because he wanted that during our custody process and he declined. So even now I have our child the majority of the time. So my question is: how did you tell your ex you were engaged and getting remarried??