I sometimes have simply bad practice sessions. These are usually the times when there is no structure to my practice, I’m not even practising (but rather playing through other pieces that I find fun), and I am tense while playing.
Needless to say, I do not feel satisfied after these sessions. Though I do, every now and again, choose to have a more ‘fun’ hour playing through exhilarating works, it’s usually not worth the short-term pleasure. The reason being that this sacrifices valuable time for a little bit of happiness, which is very unwise when considering how much more happiness can be found in the long run, as a result of practising well consistently. For me, a large part of practice is of remaining completely free, without tension, and so when I disregard this and end up playing tensely for a while, I not only feel frustrated and annoyed with the feeling of tension but also not great for slipping out of my very good habits.
I find that occasionally there are periods of up to a week or so in length when I have considerably more tension than normal. In these times I am a lot less content with my piano playing and have little enthusiasm as a result of not wanting to play with the depressing feeling of tightness in my arms. My thought here is that the occasional let-up of good habits, whatever they may be, is unavoidable, yet also quite important for one’s development since strong mental focus and work is necessary to climb out of a ‘pit of bad habits’ and back to ‘rainbows of brilliant habits’. Doing this helps you learn more about yourself and ultimately keeps the roots of these good habits even further down.
The trick, however, is to perform the mental focus as quickly and as automatically as possible so that the habits become further automated into the brain. I have encountered this situation with slipping out of habits in many other areas of life and it’s the same for all: climb back out quickly and learn for the better.