Helpppp, why can't I be nice?

okay so I'm seventeen and my dad has been dating this lady for almost two years and we get along soo well..for the most part.. She and I have a blast together but whenever my dad is around I don't know why but I become some crazy ***** and just mean to both of them and I don't understand why I try but I really can't pull myself away from this demon in my head that has a hold on me and I hate it because I love them both and I don't want to be like this.. Sometimes I think I'm just jealous because she's taking away from daddys little girl but I don't try and regain his attention ahhh