Hi @PattyK. Sorry to hear things are hard at the moment.
I don't really have an answer as to why it's possible to feel empty and umotivated from PTSD but I did want to reply as PTSD can really mess with me in lots of ways. I often feel really tired, and I wonder whether it's because it can be so exhausting simply living with it and trying, as you mentioned, so hard to manage and 'combat it'. When I am particularly tired, for whatever reason (but often stress related), I find those sorts of empty, unmotivated, depressed feelings more easily creep in and take over.
It's an up and down and round and round kind of journey for me, and when things take a turn in a way I don't want, as often happens, I can feel really sad and disappointed which lends itself to more of those depressed kinds of feelings. It then sort of kicks off a bit of a cycle for me, which can be hard to get out of. I'm not sure if that's anything you can relate to, but it's my experience with some of the feelings that can come with it.
Hearing you mention 'falling off a cliff' made me remember a picture I had saved a couple of weeks ago (I think I've fallen off a bit of a cliff in the last little while also).

A 'dip', fall, corner, etc that happens following a good run with progress, can feel even harder I think. If only recovery was a nice neat upward line!
Hoping there's a turn for the better for you soon.