You don’t. You’re highly likely going to go through some big psychological changes as you age that will probably cause perspective shifts about just who you are and what you want. And that’s biological. You’re going to experience other things that aren’t biological products that cause perspective shifts. You at 18 and you at 35 should be two fairly different people.
Hopefully you can find something that you continue to enjoy throughout life. If not it’s really not a big deal. Do what you have to, take opportunities given to you as they arise and you’ll probably end up somewhere you never expected. Work is work is work. Even if you love the field you can still hate work. It’s easier to make a change in career than to pigeonhole yourself into something that presents no way to diversify.