It's so true. I reckon most programmers get to a point in a tough project where they stand at the foot of a seemingly insurmountable mountain and think "Can I really do this?". For me, I've realised over the years that it usually happens when I am approaching burnout (even if it's in the distance) and I can take it as a sign that my brain needs a brief respite to collect itself before I roll up my sleeves.