I must confess that I have started a new HRE campaign, abandonning my old one... Yes, too challenging, and I made the bad choices. I have lost Venice, retake Bern, lost Hambourg, retake Bruges... and so on, I never progress. A few advices to the beginners, according to my experience:
-don't expect to defeat AI on the wall of your cities, AI is no more stupid, and attack with good armies. You must hold big garrisons, and it cost a lot. Defensive war is not a good option.
-After turn 20 or 30, a war with just an army is suicide. Expect a lot of elite-troops, AI doesn't recruit peasant or tons of artillery. If you want to take a city and keep it, take 2 stacks
-choice carefully your war: the map is big, and you cannot drive your armies to another front very quickly
I will apply those advices for myself... and say if I can at last seat in Rome with my teutonics.