--- Practice can be frustrating, mundane and agonizingly repetitive, but it is necessary. It's vital not because you'll improve your draftsmanship and painting or sculpting; whatever you do, it's not important. The relevance of practice for creative individuals is that it makes us proficient, this is crucial.
--- Without practice you will slave for hours or days over a mediocre scribble. If you've ever been the victim of such disheartening results, I implore you to consider the alternative. Rather than pouring your soul into the next humdrum scribble, do your ideas some justice and instead commit those hours to practicing the fundamentals of your craft.
--- Nobody is invincible, you will despair if you continually subject yourself to failure, but that's the nice thing about practice; nobody has to see, it doesn't have to be pretty. You can fail endlessly and that's actually the point; if you had no shortcomings, there'd be no reason to practice. If you can adopt this mentality, the battle's already half-won.
--- Another thing is to be reasonable, measure your progress in months, not weeks. Patience and perseverance are the second half of the fight.
--- I'm going to keep following this creed until I'm successful, failure is impossible. I encourage you to join me, good luck!