It takes a musician exactly as long to play a piece of music as it takes the audience to hear it. By contrast, it may take you ten, twenty, fifty, even a hundred hours to create a painting - while it takes the viewer a split second to have his world rocked by it (or not). This fundamental difference is responsible for a delusion that causes much of the discouragement we feel as artists, especially starting out.
We can labor for as long as we want on a painting, and so sometimes we think that if we work really, really hard on a piece we can do something that's way above our current level. We can easily see what wonderful things are going on in some great artist's work, we totally understand it, and we know that making visual art is not about manual dexterity or keeping up with the beat, so we think if we just concentrate and work really hard we can produce a masterpiece. Maybe it'll take us ten times as many hours as the master artist, but we can do it. We can take as long as we need to mix those colors, and we can rework as long as we want. We'd never make that mistake with the violin. This is our delusion. So we try, and fail, then think something must be wrong.
In all the arts it takes years, even decades to achieve mastery. A decade or two of hard work and practice to masterfully play a three minute piece on the violin, or to produce a masterpiece of visual art. And making a painting is as much a performance as playing a piece on the violin. Sure, more time on a piece may make it more polished, and we all have our good days and our bad days (and we have pictures that seem to come together more easily than others), but the reality is your level of expertise generally advances at a steady pace, and no amount of concentration can change that overnight. What you can produce in a ten minute performance right now is probably about as good as what you can produce in a ten hour performance right now (just less polished, less finished). As artists, the more skills we have under our belts the more we start to realize the performance nature of the work, and do all sorts of things to coax a good performance out of ourselves.
This talk may sound discouraging, like "don't bother trying so hard, you're not going to get any better." Really the opposite is true! - you try hard, you work hard, and you get better. You still just draw, draw, draw, paint, paint, paint. But what you should feel about your work may be different - you should not worry that you don't have the "talent", or that you'll never get it right, just because you can't make something as good as you think you should be able to right now. You can't make something as good as what you think your current level of understanding is. Again, this is the visual artist's delusion, and it stems from the fact that ours is the medium where we can spend unlimited time creating something that takes the viewer a fraction of a second to take it in.
Progress in the visual arts is very much about quantity. Some artists say, "I did lots of bad drawings before I ever did a good one." That's a good way to put it, because you learn a lot from the bad ones. But you also learn from the good ones, and even the mediocre ones. The only reason an artist gets good is s/he keeps doing it, and the only reason an artist fails to get good is that s/he stops.
Here's the kicker - that unfounded discouragement is the main thing that makes us stop, or not work as much. How crazy is that? Try to find joy in your art every step of the way, and, take your time.