Oh sweetheart I feel for you, I really do.
Things that helped me...
First, change your phone number and don't give it to him, tell him all communication in writing, lettter or email or even better thru solicitor
second, if he is visiting kids, get somebody else to do the handover... Don't see him yourself.
third, if he contacts you, wait 24 hours before responding in any way. Too many times my x made contact with me and I responded in the heat of the moment of feeling ultra pissed off, then it back fired on me because he would turn my anger at him and twist it around to make it my fault... wtf
Finally, be confident that he will give up in the end. My x is a serial abuser, he has 7 kids by 5 different women. He has told so many lies and I was devastated to find out that our whole marriage was based on a pack of lies. But the point is that, all these other women he harassed for a while then gave up and moved on to the next one.
Stay strong and look after yourself. And stay safe
x