Wait...did I just miss something or were you in the middle of the Finnish wars at the end of last video?
Oh well, luckily, the map can give a good summary of what happened.
Also, I think you're losing all those titles because of your succession laws. Gavelkind only seems to work for smaller nations. I think a ruler can change the laws when all vassals have a relation of at least 0 and you've ruled for 10 years.