This resonated with me so deeply. As someone else stated, it took me back to some of my darkest days.
What helped me overcome the steady progression into an even deeper depression was trying to understand how I could have ever believed I was happy in the first place. Could it even be considered happiness, or was that the peak in my naivety in not recognizing all of the reasons for why I should be miserable?
I was questioning everything, and while this wasn't uncommon during the worst of my depression, rather than questioning the point in doing anything in life and eventually concluding that there is non, I began questioning myself.
I already knew what people around me found joy in, they expressed it through their emotions. I began to wonder what used to take joy in and see if that could POSSIBLY hold true once again, because at that point whatever I'd depended on as my source of joy in the past was drastically reduced. Did I still like to read? Yes. Did I like tv shows? No. So maybe I can try to find one related to the same contents in the books I read. Did I like the same foods? No, everything tasted like cardboard for some reason, but maybe I could learn to make something that won't taste terrible. Did I like my friends? No... (that was one of the realizations that was the hardest to take in. We'd grown apart since I pushed them away and realized that I couldn't find a sense of comfort in them anymore because we were too different).
I was analyzing myself for the first time in my life. Kind of like getting to know myself based on who I was in the past. It was weird and frustrating. As much as I hated it, I had to start doing things. What were my goals back then? If I had to spend another week, month, year on this earth, what will I have wanted by then? And then seeing whether I could work towards it. I started off easy. Like actually speaking to my parents and finding one good thing to say every few days that eased off a bit of the pressure of feeling as though they had to call an ambulance for me again.
I went back to school. Missed an entire month and a half, but because I explained to my professors about what was going on a bit before hand and I had been getting good grades, I miraculously ended the semester with a 3.8 GPA. At the time that meant everything to me and I didn't even realize it. That despite having set up myself to fail at something else once again, I did well. So I went back the next semester. Only missed 2 weeks that time and ended it with almost a 4.0.
Like others said, keeping yourself busy helps and that helped motivate me even though there were times where I couldn't give less of a fuck about school. But I figured if I'm going to exist, I'd rather be in a classroom where people want to talk to me about whatever and something will spark something in me that may change everything, than be at home where I knew all I was going to do was stare at the same four walls I'd been staring at for my entire life.
Sorry for making this like a diary entry, but I hope you find what you need to make it worthwhile.