Practice can be frustrating, mundane and agonizingly repetitive, but it is necessary. I think, rather than being vital to improving the quality of our work, the importance of practice is that it makes us proficient, this is a crucial distinction.
Without practice you'll slave for hours or days over a mediocre piece; we've all been there. Consider the alternative, rather than pouring your soul into the next humdrum scribble, do yourself a favor and commit those hours to practicing fundamentals instead.
Nobody is invincible, and you will despair if you continually subject yourself to failure, but practice isn't showcased; nobody's watching. You can fail endlessly and that's actually the point; if you had no shortcomings, there'd be no reason to practice. Adopting this perspective is half the battle.
Make sure to be reasonable, measure your progress in months, not weeks. Exercise your patience, practice discipline.