This is a difficult challenge and it's not so much you that is the problem but something any woman would have to contend with. It may seem that a 21 year old would be mature enough to handle his father's new relationship. However, this daughter is still dealing with her feelings of resentment that were created when her parents broke their relationship, which is that of a much younger age and maturity level that she is stuck in. Her negative feelings are also probably being fed by her mother too unfortunately. So that has added to her staying stuck instead of moving on and accepting that her parent's relationship did not work out.

What is sad is that some parents are so selfish that they actually use the child to get back at their partner that chose to walk away from the relationship. The mother of this child is showing how selfish she can be by encouraging her child to carry these resentments like this.

Your partner needs to have a sit down and talk with his daughter about this behavior. If he doesn't address it then he is allowing his ex wife to cause damage to his new relationship via using his daughter. By allowing this to continue the father is allowing the mother to create a narcissistic individual and this will only serve to damage his child for what could be the rest of her life. This is unfortunate because your partner's ex is a bad influence on her own child and that's not being fair to this child. Your partner's ex is teaching her own daughter to be a "flying monkey" and to allow herself to be guided by negative and resentful and even jealous behavior patterns of others. The mother is teaching the daughter to project and distort and stay stuck. Instead this daughter needs to learn to accept and move forward instead of being like her mother who is being stuck and resentful and jealous. This mother is teaching this child to stock pile and carry resentments. If your partner allows this to be accomodated then he is no better when it comes to parenting.

What your partner needs to do is have a sit down with his daughter and make an effort to explain to his daughter that what she is taking on of her mother's resentments is very unhealthy for her.