Overworking is the bane of the watercolourist. It usually starts with an attempt to improve some part of the painting, which most times leads to then trying to fix that part because the improvements didn't work. It may stem from the feeling that whatever one is painting is the best one has done or at least pretty good and in an attempt to make it even better, ironically it gets worse. Many an artist talks about how they reached a certain point at which they were very happy with what was going on and then proceeded to ruin it.
It has been said of painting in general, and this applies particularly to watercolour that it requires two people to paint; one to do the painting and the other to take the brush away and say, "Ok, you can stop now, that's enough."