Hey there!
I just wanted to share that I'm kind of in a similar situation. Not exactly the same because of (very huge) cultural differences, but feeling conflicted about marrying the guy I'm engaged to. I'm also 23, and he's nearly 30.
My insecurities about the relationship stem from not having seen enough of the world on my own, too, just not in a sexual way. I haven't been able to settle down with a job yet because of some reasons, whereas he's at a very stable point in his career. I've barely graduated with a bachelor's, and he's already got higher education with some extra certifications on the side.
I'm somewhere between wondering why he's "settling" for me when I'm clearly nowhere near as accomplished as him, and wondering about everything I'll be missing out on after I'm bound to him in marriage (studying abroad/travelling alone, because long distance relationships, specially marriages, are not the norm here unless one of them absolutely HAS to move). And there is a constant nagging inkling about what better could have been in store for me, even when I already know that this guy is out of my league.
I keep weighing the pros and cons and still can never decide, honestly. He's very respectful, has similar values as I do, very professional and hard working... But I'm not sure if he's the one, you know? I always catch myself wondering what if later in life I come across someone else that I instantly connect with, someone who can make me laugh more than he does, or someone more impressive physically or intellectually, someone more similar to me, or more suited to me?
But then, there's no guarantee that I'll find anyone half as decent as him, even if I set out on a quest (not that there's a lack of good men, just that it's too dependent on luck, basically a sum of all factors out of our control), or any guarantee of me being able to experience everything I want, regardless of my relationship status.
I guess I'm not being very helpful here, but I would love it if you could update us here with your thoughts as you go through this phase.