I don't feel at home in my own home

So I've been with my fiancée for 17 months and he has a 3 year old turning 4 in November. I have been introduced to his daughter from the time we knew that we wanted this relationship to last forever. She knows me very well but can be very hating towards me. Now I know hate is a big word but thats how I feel she is towards me. She will never greet me when fetching her from school and would literally tell her father she doesn't want to say hello, instead she would say she is not my friend. She would do things and without hesitation, she would say it's me and would even cry should her father say we know it's her who has done those things. Now this a child I love to death and I fully understand she is just a Bby but I'm a good person by nature. I do all that I possibly can for her and more than her mother has ever done for her. Basically I love her unconditionally and because of that love I treat her like I would do my own. I've come to a point where I'm doubting myself and constantly trying to figure out what might I be doing wrong. I don't ever beat her up even though she will throw tantrums and as a result would sometimes hit her father and I. It hurts because we are trying to conceive for as long as I can remember and it's not happening, now that at this point is the least of my problems cause I have just started feeling like we should concentrate on her because her behavior is becoming worse than better. I don't know what to do anymore, I can't even communicate how much her behavior affects me to her father cause he worships the ground that she walks on. Has anyone been or is going through what I'm experiencing right now. Truth is I'm about to loose my mind trying to make this better but I come up with no solutions. I'm hurting big time. Feel free to judge, advise, I need all the help right now.