Sometimes when we ask a question, we already have the answer. I don’t think your ambivalence about getting married is a case of having cold feet. I think you’ve made a reasonable assessment of your own situation.
Often people who started dating when they were in high school just kind of slide into marriage. They and the people around them just seem to think it’s the next logical step in the relationship. Sometimes it works out fine. But sometimes it is at the cost of finding out who they are as individuals and discovering what (and who) they want in life. It’s not at all uncommon to find that the person they chose at 15 or 17 isn’t necessarily the kind of person they would choose as an adult. This realization is often painful for everyone concerned.
In your case, your fiance is enough older than you are that he probably did have the time to come into himself before he met you. You on the other hand, were in a different developmental stage. Having denied yourself experiences that are common during the college years, you now find yourself on the brink of making a lifelong commitment to someone else when you aren’t really sure who you are.
It’s not my place to tell you what to do. Only you can search your heart and mind and decide what is best for both of you. But I will venture that it is not fair to a future partner (or to yourself) to launch into marriage with significant doubts.
I wish you well,