About 30 years ago (christ, I'm old), I was able to achieve clinical death via Carbon Monoxide poisoning. It was an old car (pre catalytic converter) in a garage. Took fucking forever, because it wasn't planned out. Now that I've been officially diagnosed with ASD, I know now that what I was experiencing was an extreme Autistic meltdown from years of burnout. If I had the forethought to map it out, I would have attached a hose to the exhaust and run it into the interior of the car.
As it is, I remember the smell of the fumes the most. It took a long time for the garage to fill up with enough CO2 to put me asleep, but I don't recall any major discomfort...aside from a bit of nausea and a headache. Just sort of fell asleep, then woke up in a hospital puking my brains out 10 hours later (family came home unexpectedly, found me with my heart stopped, and revived me).
Now that I'm 50, and life is infinitely worse, I've always kept an 'escape plan' close. I have a poorly ventilated shed, a snowblower that vents in inordinately high amount of CO2, and a whole house generator (that I've disabled the CO2 sensor on) as a backup. I'm hoping that this will be enough to deliver the kind of near-painless experience that I had back in 1989. I'm a planner by nature (and trade), so I'm still working out some of the kinks to this plan.
The only thing keeping me around is a wife and daughter that I have to support. Every day, I make the calculation as to whether their lives are better off with me in it. So far, I've been able to keep myself going for their sake.
The shed is always there, though. It brings me a bit of solace knowing that I can just walk right over, fire up the snow blower and generator, lay down on a makeshift bead that I've built in there (Seasonal storage), and end this.