So what is sabr, then

Not the absence of tears. That has just been ruled out by the practice of the one who taught it.

Not pretending the loss is not a loss. The Qur'an names it as a loss.

Not silence in front of Allah, since a prophet took his grief to Him and said so in as many words.

What is left is narrower and harder. Holding on to what you know about your Lord while it hurts, and not letting the mouth say what the heart is not entitled to say. The eyes weep, the heart grieves, and we say nothing except what pleases our Lord. That is the standard, and it was said by somebody in tears at the time.

That is close to the classical definition, and the classical definition is narrower and harder than silence. Ibn al-Qayyim, in Uddat as-Sabirin, glosses the word as holding back. The self from jaza'. The tongue from complaining. The limbs from striking the cheeks and tearing the clothes. Tears are not in it.

And on the thing you are most likely to be asked for, he is explicit, and this is the answer to where is your faith. To feel the same in loss as you feel in ease is, in his words, neither within a person's power nor commanded, because Allah built human nature to tell the two states apart. What is within a person's power is holding back from jaza', not making affliction and ease weigh the same. Al-Qasim ibn Muhammad, asked what jaza' actually was, answered in four words. Bad speech and bad thought.

He reaches for Ya'qub at the same point this lesson does. Weeping and grief do not contradict sabr, he says, and quotes the verse about the eyes gone white from grief, with Qatadah's gloss on it. He suppressed the grief, so he said nothing but good.