This is an - imperfect! - transcript I made from a youtube with a talk by Zen teacher Henry Shukman.
By all means listen to (part of) it if you enjoy a charming British accent.
In it (from about the 18th minute onwards) he answers a question of someone who took part in the retreat and said that while meditation (sitting in zazen) had been going very well in the morning, in the afternoon he just felt cold and had cramps. Shukman first goes into the physical aspects, expressing how pleased he is with the blanket his wife made for him and suggesting to do stretch the leg that is cramped. Then he goes on:
"It's absolutely common that if a sitting goes well for a period of time, it will be followed by what appears to be sitting not going so well. It's just textbook. I assume that it's mostly because when it 's been going - quote-unquote - well, it feels good really, it's fulfilling, it can be very pleasant and we think: 'okay, I got through all the stuff that was holding me back from this kind of sitting, maybe even a taste of samadhi, there's many flavors of samadhi: I'm through, I've got it, ha, easy, this is nice, lovely, lovely, I've done it'. And it's not like that. We are dynamic beings, we can sure come into marvelous glades of ease in our practice, but the journey is not over.
And just because we start thinking either something like 'I've got it' or we start thinking 'man, I really like THIS', we are establishing a preference. And practice is going to turn around and put us to rights. Because if we have a preference, of course it follows that we'll have an aversion to a practice that isn't like that. When it goes back to being rough and difficult and challenging we're going "Damn it's not like it was this morning, even worse'.
Then it will be even more uncomfortable because we have resistance to the fact that it's no longer smooth and easy like it was earlier.
So what's the best thing to do? The first thing is to accept that that's going to happen. All fo that is just gonna happen. We're not perfect beings, not for a long time, who when it goes well they're just grateful and when it's difficult they're happy, they just let it be difficult.
I think that's probably how we'd like to be, to get more and more like that. More and more equanimity - untouched by when it's nice, untouched by when it's difficult.
So we really are not pinning our wellbeing on feeling good. We're not pinning our wellbeing on not feeling bad. Our wellbeing is something more intrinsic than either. That's where we're being steered. Again if I may just touch on it, of course the point of Mu is that it is a pledge of that very reality it is an emmisary of that very reality which is intrinsic to us, and is intrinsically okay. That is indeed untouched by whether something is pleasant or unpleasant. That's what we are opening up, more and more, through this practice.
At a practical level: Essentially the task is to not mind when it's difficult. Don 't be thinking 'Damn where did I go wrong? It was so good this morning, now it's not, how have I messed up?' Not at all, not at all!
It may be because you were practicing so well that the practice has brought you to something it wants you to deal with. Who knows?
Just be like a ruminant cow that just chomps through the meadow, doesn't particularly care what kind of grass it is, just keeps chomping.."