(god, it's pretty much decades by now) it's Coda, the one where Kevin reluctantly takes piano lessons. He eventually quits instead of continuing to push against his musical limitations.
Although the entire episode is excellent, it's the opening narration that I can practically recall by heart.
“When you're a little kid, you're a little bit of everything: artist, scientist, athlete, scholar. Sometimes it seems like growing up is a process of giving those things up, one by one. I guess we all have one thing we regret giving up; one thing we really miss - that we gave up because we were too lazy, or because we couldn't stick it out, or because we were afraid.”
When I was a kid myself I thought this sentiment was unnecessarily tragic. When I was a teenager I thought it was tragic yet necessary.
Now that I'm older I want to believe it's never too late, that you can still be the scientist and the artist. That you can pick up where you left off.
Otherwise, all we have left is regret and fantasy.