What should i do about him?

I was with a boy for 6 months but he made me cry everyday because he never wanted to see me or talk to me and would shout at me if i tryed to discuss something wrong with us . after a year i got back with him because i still loved him and he liked me .. now we both say we're in love again. ive dumped him about 3 times in this relationship due to him smoking weed , i found messages on his computer of him telling friends me and him will have and that ive done things with him i confronted him and he told me it wasnt him and he doesnt know who did it ... must of been one of his friends although all evidence sorta leads to him, the last time i dumped him it was due to him changing plans with me for 2 weeks to see his friends , me and him currently talk and we are alright but he wants me back but i dont know because he is going with his friends again this weekend 3rd weekend in a row and im getting so fed up. i smoked a tiny bit of a cig with him once just to try it so now he thinks he can smoke around me - tobacco and weed .. even though my nan died of cancer so im a bit wary of things like ciggarettes... i dont know what to do !!!?!?!?! please help me