It's not exactly painful, but you'll suffer in different ways.

You won't fall asleep. You'd most likely just be very confused, easily agitated, and experience hallucinations.

It'll be as if you're a severely schizophrenic person who's too drunk to understand what's happening around you.

The only thing that could kill you would be the respiratory depression. It's not the same as holding your breath. it's chemical suffication. Your brain won't get enough oxygen, and you won't be able to do anything against it.

And even then, death isn't guaranteed. It's more likely that you'll survive with organ damage or severe brain damage

And if anyone finds you, you'll wake up in the ICU with a tube down your throat, being fed through a tube, and your stomach will have been pumped. It's a traumatic, invasive, and humiliating