Jer, I know exactly what you mean by seeming possessed - they are just not like the person we knew.
The spewing eventually stops: they seem to finally run out of steam, but it can take a long while. After many years my xh no longer sees me as public enemy No 1. But it took forever, and even now, when we are in contact, I am very very careful what I say. Sad when we so much in harmony for so long.
I hope your partner comes back, but whatever happens, you will be fine.
