Worried about daughter...

Me and her dad separated 3 weeks after we had her, having been together 5 years. We got back together when she was 18 months old as I thought it was best for her at the time but I was totally wrong. During the first 18 months he only saw her 4 hours on a Sunday and wasn't very good at keeping up with it until she was maybe 15 months when he started to try and show us how important me and her were and started putting loads of effort in. So we got back together and lasted until she was 4,when we separated again (both times because of him having an affair) essentially dd and her dad have no relationship at all, he is with a new woman who has 2 daughters and he is trying to be super dad again. However my daughter (now 5.5 so again 18 months into separation) seems to be having anxiety about going to his. She cried from 2 nights before she's due to go, almost as if it's a panic attack, I've had to calm her using headspace breathing exercises on a number of occasions. At every drop off in the last month he has told me she has wet herself 3 or 4 times while with him, yet she never does this at mine. I'm starting to get a bit concerned and unsure whether I should still be forcing her to see him or allow her to make choices. I realise she's only 5 so I haven't yet allowed her to make the choice but I'm starting to wonder if there's any point putting her through this much stress when she clearly has no relationship with him. What should I do? It's heartbreaking seeing her like this. TIA