...*Thinks*
I suspect a few mistakes and one major factor.
BIG one was not doing action increasing things. I think I can chalk that one down to trying vassals in the hopes that they'd give up more actions instead of expanding ourselves.
Other was Lowlander timing- we needed to get in there while there were two FEUDING immortals, not later when Urth got his ducks in a row and could flatten us.
The last factor is both subtle and not.
Basically, in PoC, the players had a major advantage in knowing the tech tree much more solidly via the world being very close to OTL, and more importantly, we effectively had an immortal King that could try and swing away from historical bad ends. So while other nations might do the stupid every once in a while (Spirit Talkers Sacred War) we could generally keep our heads on straight...Generally.
Now though we've got someone who can do the same, and it's more of a board game, so instead of an easily distracted rabble, he can sit and plot and line his ducks up, while we tend to be more likely to chase the shiny of the day.
Now, the issue with Urth isn't quite as bad as it sounds- anti-magic of any sort will absolutely RUIN him. And again, dude likes power, he likes hoarding his power, and thus I bet if we manage to fight the slow grinding fight he thinks he can win via attrition, and save up deadlier tricks and things that would get his attention we can build our strength and Alpha-Strike him to death. And once He goes there go all his Magic's and whatever Martial he didn't lose to his newest body, all the immortality should also vanish because he's mimicking Arthryn's connection thing for that I think, and thus we come out the victor in the end.