Breakup Help

I’ve been dating this guy for 2 years. I care about him and we have a better relationship than most people; however, I know we’re not right for each other in a romantic sense. We don’t understand each others points of view most of the time. I’m an optimist and he’s a realist and that often causes too of tension for me to handle. I’m afraid of breaking up with him though. Before we started dating he was easily angered and suicidal. He saw a counselor for a while, but he never seemed hopeful until we started dating. Once at the beginning of our relationship he had a night he was considering suicide again, but his best friend and I helped him see that there was a lot to live for. He’s been wonderful ever since and has seemed very happy. I’m afraid that if I breakup with him it will ruin all his progress and as his friend I would hate for that to happen. I’ve been putting off doing it for several months for that reason. I don’t want anything bad to happen to him. I love him as a friend, but I don’t know if he’ll understand the way the I feel enough to be ok with it. How should I go about this?