Also, I thought it made the story even more powerful that the monster mostly couldn't be seen. And not only because of not needing to worry about bad CGI, but also because there was something really moving about that in itself. And I suppose it makes it even more powerful in handling the mental illness/depression, and on the other hand the ability of art to make you see more things in the world, that really were the themes of the episode.
And one of the final comments of the Doctor - that life is a pile of good stuff and bad stuff, and the good stuff doesn't always soften the bad but the bad stuff also doesn't spoil the good - that was something a lot of us need to hear. I definitely needed it now. (Not getting deeper into that because public post.)
The Eleventh Doctor is one of the best antidepressants I've come across.
Can't do more detailed commentary now, not quite done crying. But it's a good sort of crying, mostly.
Also, loved the sunflowers.