OP, I think that after reading these comments I also agree that you should realize it’s ok if you want to wait, premarital counseling is amazing, and prayer is always beneficial irregardless of what your dilemma is. You don’t want to carry this much fear into a marriage, so I think you seeking counsel and resolution is wise. Bring open with your fiancé is wise. Good for you on those points.
A helpful question we were asked before we got married was, “Does being with this person encourage you to be more like Christ?” It really helped me put my brain in the right space.
A great book too is this one by the Raineys. We went through it as newly weds, a little late lol, but it was useful! Preparing for Marriage Devotions for Couples: Discover God's Plan for a Lifetime of Love https://www.amazon.com/Preparing-Marriage-Devotions-Couples-Discover/dp/0764215477
I am sorry to see so much “advice” on this sub crossing into being patronizing. While your age is definitively not the norm for marriage in my culture, and nor is the age gap, it sounds like those two things culturally are not as big a deal for you. I do agree that you are young and should not feel pressure to get married based on age or even tradition, but I don’t think that rules out marriage. And I don’t know your fiancé, so I can’t comment on his character.
Also please know that you will be prayed for this evening, to have clarity of mind and wisdom.