> Even now, though, I continually doubt myself. The point is that this feeling doesn't go away so just try to ignore it,

No. This is the advice you'd expect from someone in their early thirties who has been coding and doing nothing else their whole adult life.

Your doubts should fuel your learning and temper your decisions. They are the voice in the back of your head and you can make it work for you instead of against.

Mastery isn't working harder, it's working smarter. It involves retraining your instincts to match your rational understanding of the domain. You get the shape of a problem and you naturally gravitate toward a reasonable solution without having to intellectualize the whole process first. If challenged, you of course have to walk back your intuition and build a step by step case, not just a rationalization, but that's fine because you've trained for that.

Study something else, anything else. Preferably with a teacher. What you're going to learn about the process of learning (and teaching) will be profound. It will make picking up and putting down new things easier, which will keep you from getting in a rut later.

Source: 40-something who spent his precocious twenties only coding. I learned a hell of a lot but I didn't know everything.