It's something that resonates a lot with me, after bumming around a lot asking "is this what I really want to do?" I eventually realised that the whole "do what you love and you will never work a day in your life" is simply a fantasy. Of all the infinite things you could be doing, you will never know if you are doing the best or most important thing. Doesn't matter. Your life is not that important. You will never be able to put an end to the questioning and self-doubt. Doesn't matter. You can learn to enjoy the questioning, and appreciate that you are one of the privileged 0.1% of humans who actually get to choose what they do with their lives.
One other thought: why do people believe they can do it overnight? I think it's because we see (what look like) overnight successes. On closer inspection they are no such thing.
I can't be the only Hacker News reader who, at the age of 19, thought "hey, that Mark Zuckerberg guy made a website that made him rich at age 19, I can make websites, ergo I can be the next Mark Zuckerberg!" But to emulate Zuck you have to look at what he was doing at age 14. Turns out he was building successful tech products for several years before he started working on Facebook. Then you get cases like Nick from Summly, which looks like a combination of rich parents, savvy investors and a desperate buyer (Yahoo). All that is true. Luck and circumstances did play a part in his particular success. But he's still a very smart guy who wouldn't have achieved what he did without intelligence and hard graft.