It's just that powerful.
Art is something you can dedicate your life to, much like a person might dedicate his or her life to family or to one particular person. Anyone who has lived in that type of situation knows the risks they run letting something (or someone or someones) dominate their time and thoughts.
Much as people can be bastards and ungrateful and spiteful... art can do the same. You might be lovingly dedicating hours, days, and weeks to the craft and art can in one fell swoop get up off its apathetic backside, stare you straight in the eyes, and tell you how worthless you are.
Art has this power to really hurt you; the crazy thing is that since we love it so much, we're powerless to hurt it back. We're just left in the aftermath of its spiteful attack to pick up our shattered egos.
The only thing that can get you through a time like this is a belief... something that exists outside of art and in your brain in a little black box (much like the airplane black box) that art can't touch. It's fireproof; it's water proof; it's earthquake proof; it's ego-shattering frank reality proof. Inside the black box is the little seed of a belief in yourself that you must have if you're the type of person who will become an artist.
Frustrating as it is, we can't really see the milestones in the pathway ahead ... "how long before I'm decent?" "When will I be able to do 'x'?" and art will sometimes do its best to cloud your vision even further and make you think that you've been wasting your time and going the wrong direction. Best to continue believing in yourself, and find mentors to guide you along the way.
Hope everyone's artful journey isn't met with such nasty things on a daily basis; even with a strong belief in yourself, it does hurt to get punched in the heart.