My partner can't accept my child from a previous relationship. Help!!!!!!?

I have been going out with my fianc� for the last two and a half years. Things were magical at the start we got on great and we were always happy. I have a 10 year old daughter from a previous relationship and I told my fianc� from the very start that she was in my life. She seemed fine at the beginning and even got on great with my child. We used to go for picnics and done stuff together. We are now living together the past year and since then things have started to go really down hill. Every time my child stays for weekends my fianc� turns into a completely different woman. She says that she is finding it hard to accept the whole situation. She says she is jealous that she will not be able to give me my first born and that she will never be able to have the "perfect" family. We are due to be married next year but this is really causing me to have doubts. I love my fianc� so much and she says that she loves me and that also if she wants me she will have to put up with this. I don't want her to just put up with this I want her to fully accept it and move on together. I don't know what to do. It's not fair on anyone involved especially my child who clearly knows that something is not right everytime she come to stay. Please help me. My life is turning upside down.