This mistake is straight up so common that one might think they would make it so you get a warning or something if you do this, at least for some of the lower levels.
So don’t worry. It’s not like you’re the first, and you won’t be the last.
My number one tip is to be careful with how many lessons you do and how fast you level up. Suddenly you will find yourself on level 13 with 200+ reviews coming in every day. The nature of a system like WK is that the workload is very light for a few levels, and then it gets exponentially tougher until you reach an "“equilibrium”. From some threads I’ve seen this equilibrium happens around level 25~ if you level up as fast as you can (I think. I might be off though). If you go slower, you will reach the equilibrium faster.
Just… Go fast for a little bit, and once you feel like you’re getting just a -little bit- overloaded with reviews, cool down the lessons.