I found myself thinking that my piano playing and composing skills had hardly improved at all in a year. This is far from true. Last year, I didn't even have the idea of composing a song and I could hardly play at all. Now I've written 15 songs or so and can play some of them fluently.
Things that are easy for me, I can't imagine being difficult, even though I know that there were. I take for granted what I can do and don't think of it as being skillful. Say I can reach for a B7 chord with my eyes closed without thinking about it or what notes it has. Well, anyone can do that, right? It's obvious what that chord is.
Secondly, I tend to hit plateaux, so I don't notice improvement on a daily or weekly basis. Then all of a sudden I will be better, but then I just adjust to that level as the new normal.
Thirdly, I have so much further to go, that what I can't do seems more significant, so I can easily discount my progress. I will some day hit a plateau where I won't improve at all without taking lessons or changing my approach to practicing to target specific skills.