What the hell do i do???

i have been with my partner for 9yrs since i was 17 and he was 22. i had a baby at 18. he cheated on me and was violent for about 4yrs then suddenly it stopped and it wasnt too bad but now ive matured i really resent him for the past, even though that stuff has stopped he is really possesive and paranoid (he smokes a lot of cannabis) and i just dont wana be with him. my daughter really dotes on him for some reason and last time i left i went bk after 2days coz she was begging to go home n see dad....we are also mixed race makes it harder....he wants to get married but i keep delaying it its not what i want, i just wish he would accept we r so different and lifes stood still for us....i would leave but im so laid back and even thinkin about the emotional stress of a breakup makes me feel sick, ive felt so low sometimes i have wished bad things on him even though i care about him....dont even know wot to write could say so much my head is just permanently jammed full of stuff ........ how do i organise myself to leave ? dont know where to start he will be devastated when i do it!!!!!!