I am having a tough time with my boyfriend?

i am 17. i have been with a guy for two years. he broke up with me last spring because he was "unhappy" and we were back together in a month. we broke up a month ago because he was trying to hook up with a girl at my school behind my back. we are back together but im afraid that i dont really trust him anymore and i cannot be happy with him.. he is impulsive and i know that he doesnt want to hurt me.. but he makes stupid decisions that do hurt me. should i give it time and see if this trust thing can be resolved.. and how do you resolve a trust problem anyways?? or should i just give up??