I'm so sorry you are going through that.
I also get those rollercoasters of days feeling hopeful and the next in utter desperation. Like you say It's a switch and feels so out of your control. I just try as best I can to hold on hard til the next one. And yeah the future dread is often overwhelming you just gotta look at your feet and stare at the tiny steps or it consumes you. The call to death is always there, some days louder, it's not silly at all and I know It feels anything but silly in those moments. This is all to say I feel you.
I will tell you what I try to tell myself and its that if there is still a part of you that wants to live, a voice of hope somewhere, try and cling to it. Cherish it whenever you can specially when it shines brighter. It's a flame. I try to keep that in mind while I accept that I will often forget it, just for how long it lasts and try to survive while I lack it.
I would not recommend you consume half a dose and leave it to destiny. That does not sound great and could get real awful. If you are unsure, If there is a part of your brain that's leading you to do it that way because of indecision, I think it is best you hold on on that. It is harder to not follow that impulse, to do nothing really, but in cases you are not completely sure I think it is not a good decision to do it that way.
I also hope things can be ok for you regardless, and that some easier days can come your way. Lots of love at ya <333