> (...) what separates euthanasia from every other tragic suicide
I see two large differences:
1. A requirement for euthanasia is usually that the patient keeps wanting it for a reasonably long period of time: it should never be a spur of the moment thing.
2. Another requirement usually is that the patient is mentally competent.
> When we hear about deaths, aren't the most painful ones those where a person has said "this human life is not worth living"?
If euthanasia is forbidden, people _will still be saying that in similar situations_, just they won't be able to do anything about it.
We usually don't want them to do anything about it - this is why we have suicide prevention. The difference with euthanasia is that you personally agree that certain lives really are "not worth living", and for practical purposes you defer that judgement, via government, to a set of doctors. This shifts the requirement away from informed consent to a judgement external to the person, from "do they really want to" to "do I really want them to".
The differences you mention (duration of ideation/plan, competence) are ultimately about determining if the person "really wants to". But this applies equally to "normal" suicide: it places the many rationally planned suicides into the same class as euthanasia (it's their life, they're freeing themselves, keep your morality to yourself).
Furthermore, according to the above link, of the 1 million who create a plan, only half report going through with it - how many with a "planned death" would not actually have gone through with it and instead endured, had they been lucid?
Should we be trying to prevent "(rationally) planned death", or shouldn't we?
I see two large differences:
1. A requirement for euthanasia is usually that the patient keeps wanting it for a reasonably long period of time: it should never be a spur of the moment thing.
2. Another requirement usually is that the patient is mentally competent.
> When we hear about deaths, aren't the most painful ones those where a person has said "this human life is not worth living"?
If euthanasia is forbidden, people _will still be saying that in similar situations_, just they won't be able to do anything about it.