This is the paragraph where the above-mentioned sentence occurs:
No one asked me to do this, and surely no one was sorry the door was closed as I strummed and stumbled along after the nirvana of these simplified songs. But the sense of happiness I felt that week — genuine happiness, rooted in absorption in something outside myself — has stayed with me.
And this is the paragraph that precedes it:
When I was 12, I disappeared into my bedroom with a $40 folk guitar and a giant book of Beatle songs, with elementary, large-type “E-Z” chord diagrams to follow. I had no musical gift, as a series of failed music lessons had assured me — it was actually the teachers who assured me; the lessons were merely dull — and no real musical training. My fingers stung as I tried to press down on the strings without making them buzz, and my left hand ached as I tried, and for a long time failed, stretching it across the neck. Nonetheless, I worked my way through “Rain” (abbreviated to two chords) and “Love Me Do” (three) and finally “Yellow Submarine” (four chords, or was it five?) and discovered by myself the matchless thrill of homemade musical harmony.
Is it possible that he means that he completely enjoys happiness of playing the songs while stumbling in it? It's like a contrast.