ahah , that's perfectly true, as well as the white page syndrome ...
best solution for me is to let the REC on while I'm jamming for ideas , or jamming on the fly on my tracks and hit REC when I'm warm enough
I think that could be the cure.
I go back to the days of the Tascam Portastudio to record demos and it would take me forever. I think because of the pressure of using a cassette tape and having limited space and what a pain it was to get it setup again might be the start of this.
With digital and the size of hard drives you can just let it roll and I should probably just do what you suggest when working on a part.
I like to record covers of some of my favorite songs (classic rock, folk, country) for my own enjoyment and to hear what my voice or guitar actually sound like. Occasionally I might get an idea for a song, but it's all for my own enjoyment and am not a professional.
But this can happen with simple rhythms also. I might play it ok and keep time, but it can loose the "groove".
I'm always blown away when I hear about artists like Brian Wilson and David Gilmour who can knock out a vocal part in 1 take. I remember seeing a Doc on Pink Floyd and David did a double for a vocal part in 1 take! No studio tricks just sang exactly like main vocal part in 1 take!