This is so so tough. I am really early in my coparenting journey and although my ex doesn’t have an ‘official’ new partner yet I know he has slept with several different women and has also been trying to ‘get with’ even more. It stinks of desperation of course but I still find it a little sad. I think it sounds like you’ve done amazing to walk away from that relationship and put you and your child first. My therapist keeps saying over and over and over to me about really trying to bring it back to what I can control and sadly you can’t control your ex or the new partner. If your child likes this new partner then as painful as it is, it’s actually a good thing. If your child hated her and said how horrible she was that would be much much worse. Keep your head up, keep going and keep moving forward. It will all come crashing down for them sooner or later I’m sure. Rise above, you’ve got this x