IDK, when I go for the "why" in it all, I find myself ruminating and going dark on myself in frustration. It's one of the pied pipers that run around in my mind which take me backward into the insanity of my past.
Instead, I try to stay present and remind myself that I've been in this situation before, that it can change at a moment's notice (at least with me - I'm okay and then down again), to accept where I'm at, and to try and figure out what I "can" do in the moment to care for myself and to show myself that I can do something (fights off hopelessness and helplessness) that might bring in a slice of light. Some days the depression is so rough, that that all I can manage is to shower or to take vitamins or to maybe even eat or to search for jobs. Anything. It doesn't have to be an entire list. I place no expectations on myself, just am appreciative of what it is that I can manage to do, if only to make the bed or whatever. Apart from billing deadlines and appointments, I keeping it simple and that has been keeping me on this side of the edge for the most part.
I might be white-knuckling it, but I seem to still be here in some fashion, so maybe this approach might work for you as well? Hoping you find some comfort, peace, and safety for yourself. VB