Some heuristics for identifying areas in which many thoughts are likely to have already been thought, in the form of questions: is this area of thought old? Is it important? Does it seem plausible that many smart people have thought about it before? Taking the outside view, am I in a position, whether because of historical circumstance or unusual credentials, to reasonably expect not to have the same thoughts about this issue than other smart people have had?

(Failure to use these heuristics is one way to become a crackpot.)

I sympathize. One of my professors jokes about having discovered a new optical illusion, then going to the literature and having the incredible good luck that for once nobody else discovered it first.