Start by understanding this... Its perfectly acceptable that you may never fully "know" what you're lifelong passion is. It's what you decide it to be, and that will change as you progress through life. Seems like you've gotten past the stage of realizing you have unlimited potential, which some people live a lifetime without realizing! Accept never knowing.
Analyze the basics, what are you good at? Think about natural talent, list the things you've always been able to perform easily and above the level of your average person. Take caution when writing the list, cockiness will make it into a book, insecurity will leave out potential breakthroughs.
Make a second list of the things you really like to do. Things that aren't necessarily easy for you, but still worth it because you get a satisfaction that overcomes the struggle. Material rewards don't count, those are merely the result of the struggle, money doesn't make the actual experience during the struggle any easier. Things you enjoy give you a rush of energy when you finally get it, because you know it wasn't easy to do!
Match up your lists, creatively think about whats compatible. People tend to enjoy doing things they're really good at, but that thing must also have an aspect of challenge that makes you strive to be the best.
Visualize.. If you only had the opportunity to combine one thing from each list and pair those two together, what, in that very moment, would you combine because they compliment each other the best. ONLY ONE THING FROM EACH LIST. Don't think about the long term, don't think about the short term, only think about what resonates most with you when you write it down. Whatever combo pulls you emotionally and suits you logically. JUST DECIDE, almost out of impulse.
HOW CAN YOU USE THAT COMBINATION TO BE OF MOST SERVICE TO THE WORLD. Don't hold back with these thoughts. I don't care what anybody thinks, the greatest fulfillment you will ever experience is that which you get from helping people. It wont be a walk in the park, but contributing to something larger than yourself brings you JOY, much much more potent than momentary happiness. Speculate and be creative imagining the infinitely possible ways that you can combine the two listed items with a service to the world.
Act on one of your ideas! Again, JUST DECIDE.They are worthless unless you act on them. In the time spent wondering which idea is the "best idea" you could have tried 10 of them and been that much wiser as to what you do/don't want to do. Experiment with it.
Focus on that final goal every single day. Write it down, study it, and go about your days in a relaxed manner, with the assurance that that possibility is already your future as if it were written in stone. Everything i'm saying hinges on your full belief that what you want is already yours. All you have to do is keep it in your mind and do your very best at whatever daily task is thrown at you, even if it doesn't seem to relate to your current goal.
Don't ever concern yourself with "How" you will make that goal become your reality. The opportunities will come to you for a change, instead of you trying to get to them, but it takes a patience level of 99 lol This part is hard...like really..REALLY...FUCKING HARD. But those who master their focus and stick to the thing they feel important end up in the history books.
30 days is the minimum...If you fall of course, you have to restart again. If you devote your very being to your idea and to helping others achieve theirs, over the span of 30 days your life will completely change. Be gentle on yourself if you slip up, that's where the growth happens. Try a different path! Just never lose sight of where you want to end up. If your lucky enough to fail over and over and over again and still be focused on your end goal, you'll be the best at what you do, and doing what you love.
Thanks to Reddit user u/imherebutimalsothere for the inspiration and statree.com for the image.