J - this is really normal. I always found the unexpected plunges down the worst, because of how they would just hit.
I used this to really help myself through:
1. I learned to say - today is going to suck, I don't have to do anything that isn't critical.
2. After losing my love to ALS I realise there is very little that is critical - drop everything and let the sucky days happen. Only do what I can do comfortably and if that is nothing, then that is how it is. (Amazingly the world did not stop spinning no matter how many days I just took off to let the grief suck!)
3. Remind myself that there was some unknown, but definitely finite number of really sucky days I was going to experience. This means that today makes one less I have ahead of me, so getting through it however I need to means it's done.
Sometimes when those bad days came they would last for several days, even a week.
As those first few years went on they length of time they went on grew less, and the intensity of the wave hitting grew less.
What I did find, is that sometimes I then would suddenly feel ok again, and realise I was feeling a bit better than I had before the crap wave hit again. I think somehow the really bad sucky times work some healing on a deep level.
However this rolls for you, I'm hearing your pain.