Scared of Being Alone?

I've been with the same guy for about 4 and a half years now, and don't get me wrong, I do love him. We've been together since we were in high school and he even choose to go to the same college as me. Now we live together and spend most of our time together other than work and school but I just can't shake the feeling of not being able to experience being with someone else. What if he isn't the one and the one for me is somewhere out there but I'm just to busy being comfortable with this guy? Am I wrong to feel this way? And yet again, there is always the"Maybe he is the one for me" feeling in the back of my head. I'm not even 21 yet and his family is already talking about grandchildren and marriage. Help..