'After a decade together, I want to date someone else'

I have been dating the same guy since we were in college. We used to be in the same class, and had the same subjects, so we ended up in a relationship that continues to this day, eight years after we graduated. The thing is, I know he loves me a lot and cares about me, but he assumes I'm going to marry him some day, which isn't true, because I don't like the idea of spending my entire life with just one person. I know this makes me sound like a selfish person, but I have outgrown him in a way, and it seems as if he lives in the past. I have changed as a woman, and have interests that no longer coincide with his. I need some time off from him, or this may end very quickly, but I don't know how to break the news to him without breaking his heart. He is one of my oldest friends, after all. What can I do?
It's perfectly understandable for him to assume this is going to move to another level if you haven't ever given him an indication, over a decade, of things going any other way. He is one of your oldest friends first, and then your boyfriend, so I'm pretty sure he will understand what you mean if you sit him down and tell him how you feel. He may be upset, obviously, but you can't allow that to get in the way of how you want to live your life. If he loves and supports you, he will respect your decision. Either way, if you don't see yourself living the rest of your life with him, isn't this a lie? Aren't you doing him a disservice too, by not telling him how you really feel? If a break is what you think will help, why not simply ask for one? It may be good for him too, because distance brings perspective.