So confused

I've recently been having issues with my fiancé due to his ex wife with whom he has a daughter. His ex wife is so mean to me and always does and says things that are negative. Her and I have never met in person, she's expressed that she doesn't want to meet me. It's confusing because she will turn around and say things like "she doesn't want her daughter around someone she doesn't know". She has texted me things like her and my fiancé are still sleeping together but when I try to call and confront her she blocks my number. I talked to my fiancé about it but he continues to reassure me nothing is going on or has been going since he and I got together. I honestly believe him, but my doubt is coming from not understanding how a woman would continue to say things like she does if she really doesn't have reason to. The other issue is that my fiancé lets her say very disrespectful things about me and him as well. She's even threatened me to him and his excuse for not defending me is he doesn't want to say anything to make her more angry because she will keep his daughter away from him. He used to see his daughter every weekend but since we have moved in together it's maybe once a month. I honestly don't know how to deal with this situation. I can't leave just because I assume he'scheating when I have no proof and he's told me he hasn't cheated. I also can't get on common grounds with his ex wife for us to have a healthy relationship. It's really causing strife in my relationship and I think that's what she wants so I really don't know how to react. Does anyone have any advice or experience with situations like this? 😕😕😕