I met DH in our late teens. I kept on dating other people -- I did not have the word "polyamory" then but I told him point blank I was not seeking exclusive! We married in early 20s. So I knew. Shoot, I lived with him first to FIND OUT.
We at 20ish yrs now. Still going strong.
There's different kinds of maturity. The physical maturity and chronological maturity you get for FREE. Just by not being dead and making it to another bday.
The rest? Intellectual maturity, emotional maturity, social maturity, philosophical maturity? Those a person has to work on to develop for themselves.
So for a young adult person to be engaged and trying to work things out for themselves and their future life plan -- to me that's great! Trying to grow their other maturities and engage in serious thinking about a major life choice.
Like I said -- can always come to the end of the engagement period and decide "Um... I'm not ready yet" and just keep on dating their person and engage again later or break it off or do whatever it is they need at THAT checkpoint.
OP's feelings and thoughts while engaged are appropriate to the place OP is at to me. It's sounds normal.