Just Go WOOOOOOOO!!!
I think sadness is an inherently transitory emotion. I don’t think it can’t last (sometimes for a good long time!) but it’s generally a response to something, and eventually it will lift.
There is something which seems like sadness, but is not quite, which I will call ‘moping’. Moping isn’t necessarily consciously chosen, but is way more internally generated than sadness, and attempts to address it are met with irritation rather than sadness' shake of the head.
Sadness can come from within too, of course, but moping is more, I guess, metacognitive? Moping is the unconscious move to self-pity. Sadness usually requires being observed, sat with, endured. It will pass when it passes. Moping will carry on for way too long if you let it.
Moping is something that can and should be fought, and generally the best way to do that, I have found, is to challenge its self-importance by (ideally with a partner) going WOOOOOOOO!!! imitating the sound of a ghost train.
Whatever the breathwork implications of going WOOOOOOOO!!!, it's certainly a very silly thing to do, and it makes it quite difficult for you to take yourself and your moping so very seriously.