Like coming the ill-considered third time, the testicles aching because there's no more seminal fluid to give. Or like retching, wishing to God you'd been able to eat something, so you'd have something to throw up. Or like sobbing when the tears are all long spent.
Like that. The spasm of grief that racked Chenrezig, when he paused from working for millenia to end the sufferings of the Tibetans, and saw that their sufferings were still infinite. When he despaired, and broke his vow, and splintered into a thousand pieces.