I'm guessing something like e.g. Alzheimers or some other quality of life degrading, but not lethal, disease that makes life (not as) worth living, at least past a certain point in the progression of the disease. But because its not lethal, it won't take you out of your misery.

I think it means you get something like a degenerative COPD that just grinds you into the ground one suffering day after another while you’re in and out of hospitals, lose your mobility, and lose your basic organ functions; no chance to recover, just withering away neither quickly or slowly.

In most places assisted dying laws only apply to people suffering a terminal disease. If you are suffering something non-terminal, you lack choice and will continue suffering. The best hope some people have is that loved ones let them starve to death.

Assisted dying, where it is allowed, is universally conditioned on the subject having a diagnosed terminal condition. If your diagnosis isn't "death within 6 months", then you're stuck with tubes, wires and chemicals, or waiting for it to happen.

So I also hope that if I get an incurable and unpleasant condition, it's a terminal one.

A quick death can be a mercy.

It's ambiguous but maybe they mean a chronic disease instead? i.e. constant suffering but no end date.

Answered in the subsequent paragraph after the end of the quotation...