i cant talk in my relationship.

My relationship with my partner has been traumatic at times to say the least. To cut a long story short, i met my partner whilst he was still married, or unhappily married. Our friendship was purely that until it became more, and after a few months my partner divorced his wife. He has two children, a son who is 18 and a daughter who is 21. I have never been involved in any affair previous to this, in fact i would of never entertained the thought. I am not writing expecting tea and sympathy, far from it, but i am experiencing problems, some people might think "well why is she surprized". However, after three years after the divorce, my partners daughter, has been in a up and down relationship with me. I have invited her over for dinner on numerous occasions, but she cannot resist making snidey comments that can be upsetting. For example, i offered her a drink, we had a Shiraz or Chardonnay to accompany the meal, and her reaction to that was that she thought Shiraz was " a pretentious wine". This is an example of her behaviour. She also invites her father out for dinner with her and her boyfriend, but not me, and the latest is sending my partner pictures of her as a child with my partner and ex wife together. Unfortenatly, although any child or teenager who has been through divorce would find it devestating to cope with, and i am totally aware and sensitive to that, i feel that my partners daughter is of quite a nasty personality, reguarly complaining about her friends, and spending money that she hasnt got, a certain 'cocky' air about her that is very uncomfortable for me. My partners son is fantastic, extremely mature, and we get on very well. I am not judging, as regards to how long a teenager/adult should take to move on after a divorce, but after three years it is beginning to wear me down. I cannot talk to my partner about his daughter without him jumping down my throat, shouting screaming and threatening to end our relationship because what he perceives as me, not liking his daughter. I think he went as far as to say that his daugher was 'perfect' in comparison to me. Maybe some people might think, "time for me to hurt". Would appreciate some mature advice with out judgement.
thanks.