One of my guiding principles is that there is no such thing as a waste of time. Time passes no matter what you do; all you can do is waste your attitude.
Another of my guiding principles is that if you are feeling crappy, just wait. Three days is usually the maximum required for a settled funk to pass, at least in my experience.
Last night when I was studying Polish vocabulary, I was having a real low point. I wasn't getting any of my flash cards right at all, and I couldn't absorb any of the audio that I was listening to. Sometimes you are just having a crappy time. I don't care what kind of cheerful optimist you are, it doesn't help to be in denial about the fact that sometimes you feel bad. We all sometimes feel yucky. You can't paint over it with optimistic aphorisms. But that doesn't mean you have to drown in doom and gloom, either.
There's a technique sometimes used in meditation where one does one's best to empty the mind of any thoughts. Sometimes one can't empty the mind. There is a Buddhist term called "monkey-mind", referring to a state that is so agitated and unsettled that thoughts are just jumping all about like a frenzied caged monkey. One strategy for dealing with this is to acknowledge and embrace each thought as it bubbles forth, and then release it.
The reality of the human condition is that it is not always one level steadfast line. There are ups and downs; there are peaks and valleys. Things might move in a circular fashion or in zigzags rather than in a linear fashion. The optimal way to handle this is to realize it and to deal with it. Know yourself and how you react to what happens.
The simple truth is that there are times when we will shine, and there are times when we will stumble. Last night, for me, was an emotional low point. Maybe my neurons had just fired enough and they needed a rest; who knows. Instead of continuing with pushing myself uselessly, I just watched a Polish comedy with English subtitles on YouTube (who knows, maybe in a month this link won't even be there). Actually, I didn't even watch the whole thing; I fell asleep about halfway in. And I wasn't concentrating on trying to understand the Polish, but rather, just relaxing and absorbing the entertainment value of the movie.
I find that there are times when I'll absorb a high percentage of material that I am studying, and there are other times when I'll absorb a low percentage of similar material. Sometimes there are things I can do to improve my receptivity to study, and sometimes not. So if I'm in a place where I'm not gaining any value from what I'm doing, and I'm not able to improve conditions to the point where I am gaining value, then at that point, if I continue, I am truly wasting my attitude.
One thing I like about Anki (a spaced-repetition program that I have raved about in prior postings) is that it gives you statistics about what time of day the success rate of your study is the highest. For me, the statistical information is really striking--around 11 am, 3 pm, and to a lesser degree at 10 pm, my percentage of right answers is dramatically higher than at other times of the day. I've used that information to try and make sure that I get at least a little bit of study in at or around these apparent most optimal study times if I have the opportunity and the inclination.
And waiting it out works. Though I was close to zero percent last night, this morning when I studied the same material, I was at around eighty percent, and this evening, my percentages have been about just as high.