a) If you can't see how to make this work, quit.

b) Beware of rationalizations for quitting when you're just in a local trough emotionally. My heuristic: I don't stop doing this until I find something better to do. That forces me to find a constructive reason to quit.

c) The worst possible outcome isn't failure and quitting but ongoing mediocrity. Either because you lack the courage to quit, or because you get just enough feedback to keep you interested. This can be arbitrarily hard to detect, but for starters keep monitoring your metrics and looking for opportunities to revise.