I think part of that is the fact that the better you become at something, the better you are at recognizing how much you don't know; and I think that that skill generalizes, so over time you can get overwhelmed trying new things because you can "instantly" see how much you're missing, especially if you have no transferrable skills that apply.

I definitely get this with speaking foreign languages - English feels like a second mother tongue to me at this point, but even though I can create Spanish sentences, I get choked up when about to speak.