Ugh >< contrary to some of the above posts, I think my art did get worse at one time to be honest. That was several years ago now.
It mainly had to do with straining myself, and forcing myself to get better at drawing. I was going through one really bad phase of "my art sucks". Even though I practiced a lot, and repetitively, it sucked... and it got worse compared to my works the year before. So of course I grew frustrated, and doubted my abilities. Anyhow, I took a bit of a break and went back at it with a clear head - focused on the fundamentals and practiced patiently. Lesson learnt everyone, practicing a lot is good, but you need to practice patiently and positively. And a short break here and there never hurt anyone!