How do you know whether or not to give up?

After 3 years of this roller coaster I don't know if I should just finally give up and leave him or stick it out... I've only stayed with him because of his daughter, I'm the only mother figure she has had and I have taken her on as if she's my own.. I love her beyond anything in the world. She has made my heart feel complete and I can't imagine my life without her.. She calls me mommy and she's my little partner in crime we do everything together.. I know if I leave he will just have random woman in and out of her life.. when he cheated on me he even brought his little slut around her and confused the hell out of the poor baby :(..

I don't know whether to stay for the happiness of the child and to try and give her some stability or walk away and try to move on.. her mom walked away and turned to drugs and is in prison due to drugs. I don't want to be that person that walks out on her.... :( :( I'm so heartbroken and torn.