At the beginning, I thought this was written by someone like me - i.e. the not clinically depressed person in a relationship. I'm impressed this person had the insight and compassion to understand what the people around him went through, and thought, and the strategies they composed, while trying to rescue him from it. For me, ten years of watching the love of my life struggle with severe manic/depressive episodes, there are a few more bits of advice I could add for the partner of someone with depression, but everything he says here is accurate. Specifically:
* You can't just entertain them with novelty; it doesn't last
* You can't drag them out of bed, or of the darkness
* You can't argue or reason it out - you'll end up depressed yourself if you try.

What you can do is internalize their reasoning and find that you don't accept the ultimate, inescapable negativity that they see everywhere. You can be unreasonably kind and forgiving. You can never lose your temper. You can see the things you know they would love to see in the world, if they got out of their hole, and you can gently try to put them in the orbit of those things and point those things out to them every day, to get their internal excitement and curiosity about the world spinning up again a little bit.

There are so many things you shouldn't do, the list would be too long to write here. But the number one thing is not to try to out-depress the depressed. It's good to hug them and to be there and to go into the darkness with them. But if you take this on, you can never let it infect you. If you're in love with a clinically depressed person:

* You can never have a bad day.
* You can never tell them that you, too, question the reason we're alive or the meaning of the universe.
* You can never respond the way you would with a normal, self-regulating person, when you shoot the shit about how fucked up everything is. Or even some specific thing. Like how fucked up some twitter thread is. You can never let something visibly get to you, because your depressed person will take that as a cue that it should really get to them. They aren't just depressed; they take cues from what's around them and they will be more depressed the more depressed other people are.

For this reason, it's urgent that you pretend like everything is fine. You have to understand that clinically depressed people take more than they give. This is okay because they aren't doing it to gain power or to hurt anyone. Being able to give and adapt to them makes you a stronger person; it forces you to be a superhero, no matter how much you don't want to be. No matter how much you want to get depressed too, you know you just can't because someone's relying on you. It makes you better at coping with all the hard parts of life because, if she can't cope, someone has to cope, and the only someone is you. But it's hard. And you'll rarely or never hear a heartfelt moment of understanding / apology like the one in this post. You have to know that you're doing good, without anyone else ever telling you that you did a good job, or that it's going to be okay. And with most of your friends and family telling you to get away from the crazy person who you love more than anything.

You'll be alright, person who loves and supports depressed person. It'll make you stronger in the end.