Firstly, thank you for starting this thread. It's always hard for me to admit when I'm "tanking" (my word for when it feels like I'm sliding emotionally); it's been a long time for me too and I keep waiting to "feel better." Fighting against your own head means you are fighting your own most formidable adversary. After all, no one can make me feel as crappy as I can, even without realizing I'm doing it. There are still the same two years that I can't remember, I still take meds and wonder if there will ever be a day that I feel like they really do something. The night terrors, the nights I'm too afraid to sleep and the ones where I pray to sleep and can't all bleed together.
I've been diagnosed for years (15?) and agree with the idea that the moment-to-moment psychological torture that I do to myself when I'm sliding is the worst. Here's how I deal with it: I am not in control of my symptoms, but I can fight their emergence with therapy, meds and diet (lol- no caffeine after midnight. Genius, huh? :) ). In those moments where I'm feeling useless, suicidal etc., I try to remember that these are direct challenges my own brain is using to "trick" me. Instead of a stomach ache, my brain sends me a message telling me I'm useless. If I could control my stomach aches, night terrors or any of the other symptoms, I would have. It's up to me what I do with them in total and recognizing that perhaps the thoughts that feel the worst aren't as "real" as they might seem- does that make any sense?
Somehow, that helps me in these moments.