Volume
Most great artists are prolific. If you make one thing and then try to figure out how good it is instead of just making more work, you are probably holding yourself back.
Imagine you have a year to make a painting for an exhibition. You could spend the whole year working on one piece or you could start a new painting every week and then at the end of the year pick your favorite for the exhibition.
There are 2 reasons that the second method is better:
It is highly likely that you will end up with better artwork by picking the best painting out of fifty than by only making one.
At the end of the year, if you have made 50 paintings you will almost certainly have made yourself a lot better as a painter than if you only made three.
In fact, even if you were able to make a better or more impressive individual piece by focusing for that long on a single painting, I think the skills that you would learn by consistently putting out work would be far more valuable.
My process has been a lot closer to the first method than the second than it should be over the last few years. I hope that by increasing the volume a bit on my work, I can make myself a better artist.