Years ago, I watched a television documentary on one of the specialist music schools. During a lesson, the music teacher was teaching a young lad, and issued the instruction “when you practice this exercise, if you are not in the mood to practise with full concentration, then please, I would rather you didn’t bother to do it at all”. This was in relation to an Oscar Beringer exercise to work on a particular point of technique. I have no idea whether the pupil did the practice properly or not – but it is a good point. Half-hearted practice can often be worse than no practice at all.
I am everlastingly grateful to my piano teacher, who used to set just half-a-dozen bars of a study, compared to my cello teacher, who used to set three pages of grindingly dull Grutzmacher. Now, of course, I realise that string players (and presumably other instrumental players) have to develop a different kind of muscular stamina, to be able to wield their bow or support their instrument for hours at a time. And grounching through all the endless semiquavers also develops the mental stamina to stay the course.
Still, whatever you are doing, be it practising or knitting or cleaning the windows – if you are not in the mood to do it properly, then you are probably wasting your time – at best – and possible creating more work further along the line.