I can understand your frustration. I also learn a lot better when I have some sheet music in front of me to reference. It’s still pretty hard then because it’s hard to keep up with the speed of those great players we want to sound like. Without a teacher it can be pretty hard to get access to examples of pieces played slowly.
I think others here have said it before, but I would say work with the materials you have for now. The sheet music in the book may include songs you’re not very interested in playing, and the songs you do like won’t sound like the versions you aspire to. This is inevitable. But just like with any instrument, you can’t expect to be able to start with the most amazing pieces. You don’t start learning piano with Chopin’s Fantasie Impromptu, you don’t start learning guitar with Stairway to Heaven, etc. You start with Mary Had a Little Lamb and Hot Cross Buns. It’s a process.
First, you have to teach your muscles how to perform the required motions, and this works best with simple exercises and techniques. This is usually done through simple pieces. Then you have to build up your repertoire of techniques and phrases by learning a variety of slightly more complex pieces. If you’re working within a particular style, you gradually pick up its nuances, its common phrases and runs. With this combination of physical ability and understanding, you become able to play more advanced pieces.
Jonkara Bushi and Aiya Bushi, especially the way we aspire to play them, are very advanced pieces. Attempting to jump right into them can be very intimidating and frustrating, and I think you’re feeling that. Like others have explained, not only are these pieces difficult to start with, but most players add their own touches to the piece. This can mean anything from adding little flourishes to restructuring the entire piece. They’re also often improvised, so you’re just not gonna find sheet music of it anywhere.
What people have said is work on understanding the foundation of a piece, the original folk song version. If you understand that, then you’ll be able to see what people are basing their improvisations on, and you’ll be able to better pick apart what they’re doing. However, you also need strong technical ability and familiarity with the style. Without that, it’s almost hopeless.
If you start with the sheet music in the book, you will build up a repertoire of techniques. Then later, when you listen to a great player, you’ll be able to go “Hey, I know what they’re doing because I’ve done it myself!” Even when you hear something completely new, you’ll be able to break it down.
Maybe you already know all this, but I thought I’d lay it out anyway. Relax a bit, man. Work with what you’ve got before you jump up to the crazy stuff.