"I was haunted by the possibility that the thing had no answer at all, and by the even more dismaying thought that the damn thing was simple and I just couldn’t

"I was haunted by the possibility that the thing had no answer at all, and by the even more dismaying thought that the damn thing was simple and I just couldn’t see it. Am I stupid to keep trying or stupid to not be done yet? Either way …" This resonates so much with me. This is the biggest gulf between doing very hard assigned problems and original research. Just knowing that a problem is 'at your level' really helps me with solving it. Any failure becomes a temporary setback, rather than an indication that maybe I won't be able to solve this. I've tried to meta-game myself on this, simply by assuming the solution is achievable. It gets ever harder the longer a problem goes unsolved though. Simply because it takes more effort to ignore the 'evidence' suggesting the problem is hard.