It is so easy to fall into the trap of impostor syndrome. We all have these problems, and coupled with the potential complexity of some programming tasks it can feel overwhelming.
I started using the pomodoro technique, and I brought up the fact that I felt unproductive because I was having a lot of trouble achieving my goal of 11-12 pomodoros per day.
She was like, "you realize 11 is a very hard goal, right? When I was using pomodoros I was usually doing 8 per days. Most people struggle to do a single one when they start."
It was illuminating.
In grad school my adviser was seen as the most productive, intense person in my department. We got on pretty good terms, and when I was struggling he started telling me about how much time he spends doing e-mail and reading random books (if you paid attention you might notice the arts and philosophy books laying around on his desk; he's an engineering prof.)