Heer loved Prem unconditionally. She literally gave him her soul if that makes sense. The way she sees it, because of a misunderstanding, he slept with someone else to get revenage and that for her is beyond hurtful. I guess she's upset that he never cleared it up with her before commiting that sin. After she left Prem, she had no contact with Prem and had no idea what he was doing and how he was doing. Maybe she imagined him living with Ashlesha or someone else. Whatever it was, she was angry and for 6 years that anger built up and now she finally see Prem and I guess she is taking that anger out of on him that she was building up for 6 years. And like the orignal poster said, Chahat is all she got and she knows Prem is a wonderful person (how she defended him when Chahat called him khadoos) and she knows he would be a great dad to Chahat and he can give Chahat a lot more than she can (fiinancially) and she's scared that than she'll lose Chahat as well. AT this point, she can't see forgiving Prem though obv. later on she will.
I also see Prem's point of view. Heer thinks she has commited the "sin" intentionally but of course Prem know he hasn't. He knows how much he loves Heer. He knows how much he loves Chahat. He knows how much he wants to be part of both their lives. I don't support adultry and I think its an unforgivable mistake but in Prem's case, he didn't even do anything. But what he did or didn't do to Heer, doesn't make him any less of a father to Chahat. He deserves to have Chahat in his life. Chahat deseves to have him in his life.
I think the truth needs to come out, not for Prem or Heer's sake but also for Chahat. I think by promising Heer that she'll never talk to Prem again, she won't be happy either since he has been the only father figure in her life and she lost him too.