I recently went through a few weeks where I wasn't painting much and wasn't feeling good about the art I was making. As any creator knows, this is a pretty common occurrence, going through these kinds of "down" periods.
After half a lifetime of being a writer, and a few years now of being a painter, what I have learned is there's only one way out of these kinds of slumps: paint (or write, or whatever) your way out of them.
Most of the time when I'm feeling negative or less than enthused about what I'm making, it's simply because I'm not doing enough of it. In other words, I'm rusty...and I'm not working enough to get in the groove again. Thus, as difficult as it may seem, when I'm in one of these periods, I simply double- or triple-down on my production. Even if I think it sucks.
There is something about just getting moving again, creating at an increased speed and output, that helps me shake off whatever creative depression I'm in.
This is why it's helpful to work on paper sometimes. Because it feels less "serious" or permanent than working on a canvas or surface. I start to scribble out some ideas, make a few dry-runs here and there, pretty soon I am back to feeling good about my skills and what I'm creating.