Writing stuff that has already been done and not knowing.
There's so much out there that I can't possibly know what's already been done. A year ago I started defining three characters in preparation for a proposal, then I discovered it was almost exactly the same as The Big Bang Theory. I had to abandon it of course.
[i now love BBT of course, superb writing]
How do we cope with that? I don't suppose there's anything except suck it up and start again