What would you do in this situation--break up with him after all of this?

My best friend/ on-off boyfriend of 4 years wants to marry me one day. I can see myself marrying him. I love him deeply. We've been thru a lot together. But we are only 21 & the thought of NEVER BEING SINGLE AGAIN in my life scares the pants off me. I won't do it. So, is it reasonable of me to want to "hold off" such a serious commitment? Even if we don't get engaged for years, we would still be dating exclusively until then, with marriage looming in the future, and then that would be it forever. So???