What should I do? We are always arguing and it is bringing me down?

My partner and I have been together for a year, he has two kids as do I . We are always fighting, over everything, I get mad that he is so pessimistic about everything and always finds the worse thing rather than the best thing, he does not support me but is negative instead. For example I was going to walk to the shops rather than catch a bus and he says "yeah sure, I bet you don't." ( I am trying to lose weight) Anyway we just always at each other, he has such a short fuse and he takes things so personal eg: I said "I hate the way you are so negative" and he says "Oh so you hate me now" totally twists what I say around. He also sulks alot and swears at me when his mad which I have told him I hate. Lately we have been arguing in front of the children as he tends to drag them (his two 4 and 6 years into the fight) I find it childish. When he is loving and in a good mood he is the kindest and nicest partner, very thoughtful but lately its just fighting and he thinks I antagonise him which I am not aware of. What should I do?