Do I have commitment issues?

I've been in a relationship for four years. However, when I think about walking down the aisle and promising to spend the rest of my life with him, I suddenly become overwhelmed with horror. The idea of doing this petrifies me and I can't figure out why. I've been wrestling with this thought for some time now and it has, for the lack of a better way to say it, been keeping me awake at night. Especially because many of my friends at the fossil age of 23/24 have started picking out rings and some have already gotten engaged. This situation has even led me to question whether I've been wasting my time being friends with the wrong people because as far as I'm concerned no one should want to get married so early. So my question is this: Am I the one with problem? Do I have commitment issues? If not, then is it the person that I'm with? Is it possible to envision being with someone for the rest of your life and actually be happy with that? I need some help here. Advice?