<human>: My boyfriend and I have been together almost four years now, but I'm getting to the point where I feel like I could be happier if I were to end the relationship. There is nothing catastrophically wrong with our relationship, but I just feel like it has fallen flat and I don't know what to do.

We got together when I was in high school and I had never been in a serious relationship (or really any relationship) before then. We had really great chemistry for a long time, but I feel like it has fizzled in the past year or so. I realize that long-term relationships can become less intense, for lack of a better word, over time. But I have nothing to compare this relationship to, so I don't know if this is how all relationships get. 

I really do love him, but I don't know that I am "in love" with him anymore. I know it sounds cliche and stupid, but I don't know how else to explain it. I find myself wondering about being with other people far too often, and I feel horrible about it. I know he loves me very much, and we have been through a lot together. 

I'm terrified that if I do end things with him, I will realize that I have  made a terrible mistake and that I was just taking our relationship for granted. However, I don't want to stay with him and feel resentful because I didn't get to have any other experiences. 

He is my best friend, and frankly, I don't have many friends. I know it is so unfair to him to stay in a relationship where I'm not appreciating him as much as I should. I'm terrified if I break up with him, I will lose him from my life completely and I don't know if I could handle that. And most of all, I don't want to hurt him. I'm so at a loss for what to do at this point.

Can you condense the text into three sentences or less?
<bot>: I'm not happy in my relationship anymore, but I still love him and am terrified of losing him if I end things.