How do I get over my fear of breaking up with my boyfriend?

I love my boyfriend. He is my best friend, and we have such a great time when we are together. Lately, I have been feeling that we want different things out of the relationship, and he is not prioritizing our relationship over other things in his life. I hate being unhappy in the relationship, but I am scared to break up with him. We do have a lot of good times, and he is a good guy. I want it to work; I just don't know that it can. I am terriffied to leave the relationship and be on my own. How should I handle the situation?