@ghost
This sounds very awful and I offer my sympathy. I've been through a similar situation, but made a choice which I know is hard for anyone who isn't a cat or a robot.
One of my best friends. When things are good they're great, but every 2nd or 3rd game night it's just this massive black cloud that's REALLY difficult to navigate.
I can't speak for your feelings, and I know that there is more to the story as to why this person is one of your best friends, but I can't help but feel that you are telling yourself that they are so close in order to justify the guilt you feel in wanting to tell them off. Further, I think you want to justify why you feel like a bad friend even though you're not the one acting badly. Said another way, you may consider them a friend, but I don't think they are treating you like one.
I offer this anecdote.
My best friend back home was my first girlfriend. We have known each other for over 25 years. (Fuck, I'm old.) She got married, had her first child, and then was struck by nasty post-partem depression. During the course of her treatment, she developed mania and severe anxiety. She didn't tell me a thing about it for two years, during which she never returned my calls or texts. But when she did finally reconnect, she explained simply: I didn't talk to you because I didn't want my problems to be your problems. I disagreed with her strategy, but I can understand that what she did had good intentions.
I'm not seeing that here.
I want to approach them as a brother and recommend counseling and seeing a doctor about it, but I'm afraid they'll go super edgelord about it or storm off out of the friendship out of "chase me" rage.
I will borrow from Hagakure here, as I do these days:
"There is something to be learned from a rainstorm. When meeting with a sudden shower, you try not to get wet and run quickly along the road. But doing such things as passing under the eaves of houses, you still get wet. When you are resolved from the beginning, you will not be perplexed, though you still get the same soaking."
If your friend wants help, they will listen; if they do not, they will not; so there is nothing to do but accept that the result is actually out of your control.