Trouble with the boyfriends daughter

We have been together for 3 yrs. Just recently his daughter that just turned 12 has started lying and having a really bad attitude towards me. I have bent over backwards for this kid to include her in all aspects of our life. I spent more time with her than her own mother or father does. He works a lot and I would go pick her up when she was supposed to be at our house when he was busy . I have been the first person to do many things with her like baking cookies , planting flowers and other simple things that a PARENT should do with a child and I did with my own daughter who is now 20. This 12 yr old has now after 3 yrs decided that she doesn't like me and is telling me lies on a stupid childish level things just to hurt my feelings or things that she knows will upset me towards her fathrr. I am not a person that likes to deal with children and things like this that come with them. her father just saw for himself what I have been telling him on saturday. She told me to shut up and told him I had screamed at her when it was an obvious lie because her was standing fight there and knew that I hadn't. She also said she hates me and I get on her nerves , I think what hurts me more than her words is her father and his reaction to the event. Yes her corrected her and had a talk with her but he also told me to shut up right in front of her and I was infuriated I told him that is why she is treating me that way because he shows her how. After everything was said and done he and I had come to an agreement or so I thought we had. The girl has never had any responibility as far as chores or aanything at our house but does at her mothers. I thought we had come to the conclusion that she was to now do the dishes after dinner and clean her bathroom. Well the time came for her to do dishes and when I mentioned it he said he would do them insteadd, what a slap to my face. So this has caused a lot of resentment for me. I ended up doing the dishes and cooking the dinner and sweeping and mopping the whole house and doing laundry , he mowed the lawn and she did NOTHING all day , she stayed in her room and watched tv , I guess the princess needed to rest. So my question today is should I just leave the situation and move on ? I am very hurt by them both and with her attitude towards me and his lack of repect forme or my feelings or worth as a person I am feeling like I should just throw in the towel and be done.

Sounds like if she changed suddenly her mom is in her ear. Girls develope a sense of loyalty to their moms when it comes to dads girlfriends. I know because I did the same crap. Pretended not to like the girl friend so much because my mom didn't want me close to her etc. I would back off that situation, meaning leave them be for a while. If he wants you to come back into it, make clear your expectations out of him and her. If he doesn't want you back in its his loss...devote your time and energy else where. They need to know you demand and deserve respect no compromise.