I came across this video, and thought of this thread:
It's part 5 of a series, I'll probably binge-watch it later. There seems to be more in the series after it as well. These are all people that
thought they were great at martial arts, and instead didn't have the fighting ability. Most of it is newbies coming into a gym and thinking they know everything. The #7 one was a guy who was a "street fighter" with a "21-0 record". He went into a BJJ gym and challenged the instructor to a boxing match, and still got his butt handed to him.
I also think of the purple belts at my school. That's got to be one of the hardest classes for the kids. Our school separates the classes by belt level. When you start, you're in the white & yellow belt class. For a lot of the kids, the most advanced stuff they see before they get their purple belt is the yellow belt stuff, which honestly isn't much more than what the white belts do. The white belts have the basic punches, kicks, and blocks, and then yellow belts get a very simple form and a very simple set of one-steps (each one is just a block and a punch).
They leave for purple belt without realizing that they'll have to learn back kicks, how to do footwork with their kicks, spinning chops, and more complicated forms and one-steps. We've had several kids leave crying in the middle of purple belt class, because they get overwhelmed with everything they have to do. They felt like the king of the world; like they knew everything there was to know about Taekwondo. And suddenly they get into a class where they don't know anything on the curriculum. Usually a week or two and they get the hang of most of it. That which they're still struggling with, they at least are confident enough that they can learn it. But it's a huge shock when they realize there's a lot more to learn. And that's just at purple belt.
By green belt (the next time they switch classes), they're older, and they've been through this experience already. They usually handle this transition much better.
But this goes back to the points made to the OP. You don't know what you don't know, and if you're training by yourself, those things that you don't know you don't know are the things that will hamper your training.