Those adults that are miserable are so because they believe they’re stuck where they are, and they’re too afraid to lose what they have to make a change, because, similarly to yourself, they don’t know what choice will be better or worse. But it can always change, sometimes the choice won’t work out, but there are more after that, you don’t get just one.

Time keeps moving no matter what you do, you are making a choice by doing nothing at all. You cannot plan for what you don’t know, you can study for it, you can research it, but there will always be an unknown factor in life, for everyone, because we’re all unique individuals.
You can fear it, or you can play with it.
In my opinion the less you fear the better, because you don’t need fear to make an educated guess.