The drama seems to follow me no matter what

I (37M) got divorced 6 years ago from my wife (39F) and recently got engaged. (37F) My ex hates my fiancée and from what I can tell she is just lonely and maybe a bit jealous. I have always been told that I am a catch, good looking, very nice and a great dad and I’m guessing there may be some regret there as she left me for someone who was/is married and it didn’t work out. It has been fairly high conflict for the first 5.5 years and only started to improve this spring when my ex and I were forced to spend a day together at the hospital due to an injury for one of our children. We have been able to be around each other at extra curricular activities and it’s been nice not being anxious every time. Recently she has not been herself, she has been telling me how kind I am and has been very interested in my engagement and has been making comments about how she thinks I can do better, etc. Not going to lie, I did have some feelings of a possible reconciliation which would be highly unlikely but I’m a guy and I want to be able to see my kids daily. This came to a head last night when she called around midnight to confront me about something I did after our divorce, after I started talking to my now fiancé but before we started dating. I had a night with someone who was a mutual friend and was also talking briefly to a coworker of hers (I know, very stupid) the coworker and her didn’t get along and she actually pursued me but nothing came of it besides a week of messaging. She is now threatening to tell my fiancé in hopes of breaking off my relationship which would most likely be the case. Just needing to get it off my chest, I can’t tell my fiancé any of this but this drama seems to follow me and I can’t catch a break