Not the person you responded to, but as you practice an art more and more, and also as you consume it as someone enjoying it or interacting with it, you begin to have an "aesthetic" as a consumer of that art that slightly outpaces your skill as an artist in that art.

The trick is not to become discouraged when you can SEE you suck, but instead analyze the best you can HOW you suck and then take action to change that. Don't dither around thinking, "Oh, I think this is bad but HOW DO I KNOW?" You don't have to kowtow to some God of Technicalities with every twitch you make. Make a goddamn decision about how you suck then do something to fix it. Trust in yourself.

So, like...say you can play a piano piece in technically accurate way. The notes are right, the timing is right. But it has no soul. It's flat.

Why doesn't it have soul? What is soul? Well, listen to a lot of pieces you think have "soul", and pieces that don't, and you'll eventually figure out what the difference is caused by.

The trick is to understand that progress is incremental. You won't wake up a master of whatever thing you're doing overnight. Instead, you find one thing to learn, to do better, and as you learn to do it better, you go find another thing to fix. And all those small, incremental bits of learning pile up.

As for selecting what song to work with--personally, I think people are motivated best when trying to replicate things they already love. That's how I self-teach. Although, that said, I'm not a music teacher so a real music teacher may have different opinions here when it comes to music, as there are pieces that are technically harder to do than others.

But when it comes to something like writing? Absolutely, write what you want to read.

Source: I played violin a few years, but I'm also a fiction writer and I will take books that I don't like and try to figure out why I don't like them. Then I try to avoid doing in my work what was done in the "failed" works.