Resenting stepdaughter?

I have been with my boyfriend for 3 yrs. He has a 5 yr old daughter who comes over every wkend. We have a 2 yr old son together. Well when we first got together she really liked me. She wld go home and tell her mother she wNted to come back over and c me. Well i got pg and it changed. Flash forward to present and just this weekend i had a talk with my bf about it and he basically said it was in my head. Butabout 2 wks aho her mpm calls him and says that the sd told her that i have been choking her... NEVER... I wld never do that!!! And the bf beleives me. But this weekend she wanted a damn banana and she cldnt open it so i asked her to give it to me and she dowsthen she runs to my bf and crys to him that i wont let her have it. She does gt in troble by her dad. Then i was gone all day saturday and the bf has both kids all day and as soon as i get him she becomes his tail and tags along everywhere. If he came in to talk to me she followed. Well i asked him to make her stop an he does. Then our son runs over and jumps on the couch where his dad sits and she was sure to take it over by proclaiming thats her spot. Well som may say IM jealous. But jealous of what. I have my bf l week. I think i cld share for 2 days. But it is irritating to have to have her every wkend. And if we wanna plan something and he doesnt get her then her mother makes him feel like the s of the earth. Well basically im growing to resent her an im terrified that ill never be able to change her feelings or my own and it will affect relationship wih my bf... Help!!!