When is euthanasia acceptable? Where do we draw the line ethically?

I recall recently seeing an article posted that was related to euthanasia, and I started thinking about the subject. I see both potential pros and potential cons associated with it. For example, there's the concern about family members or authority pressuring an ill person to opt for doctor-assisted suicide to ease financial burdens, for instance. There's also the benefit, on the other hand, of allowing someone who is terminally ill or guaranteed to live the rest of their life in excruciating pain the option to go out on their own terms. With proper oversight and ethical considerations, it generally seems to be an all-around ideal to provide an "opt-out" for those who would only continue to suffer and would rather not prolong it, as a merciful alternative to forcing them to live it out.

But then there are some trickier questions.

As a disclaimer, I spent nearly a couple of decades struggling through depression and have been surrounded (and still am surrounded) by people who struggle with their own mental illnesses. Because of this, I'm perfectly aware of the stigma and subpar treatment of mental illness in general. With that in mind, I completely recognize that there are certain conditions which are, at this time, completely untreatable and result in peoples' quality of life deteriorating to the point that they become perpetually miserable, particularly with certain neurodegenerative diseases.

Thus, the question occurred to me: wouldn't such a condition be the mental health equivalent of a terminal illness? Would it not be unethical to force someone to continue living under conditions in which their quality of life will only diminish? Shouldn't someone who has such a condition, and is either of sound enough mind or with a written statement of their wishes from a time when they were of sound enough mind, be able to make the same decision about whether or not to opt to go out on their own terms?

And yet, as reasonable as it sounds, for some reason the thought of it feels wrong.

Is there something fundamentally more wrong about euthanasia for mental health vs. euthanasia for physical health? Is it just a culturally-learned ideal?

More importantly, what makes euthanasia acceptable in some cases and not others? Which cases do you think exemplify the divide? Is there something more fundamental that we can latch onto? Is there a clear line we can draw? Is psychology itself just too young a field for us to be drawing that ethical line?

I'm genuinely not sure how to feel about this subject. I would be interested in hearing some other thoughts on the subject. The questions above don't necessarily have to be answered, but I thought they could be good priming points.