Bamff raises a good point.

You pretty much have to have a "real army" in each region. The AI is going to jump on weakness; no problem as long as you can retreat your garrison and know that relief is 1-2 turns away. Also, you may want to have a siege army near trouble spots; repulse the invasion and move on your foe/siege and assault before the pope can tell you to stand down :)

Take a close look at the map; is there any way to reduce your border area either through retreating or advancing; reducing exposure allows you to consolidate garrisons. Do you have an opportunity to remove a problem through eliminating a faction?

The advance through Central Europe can be hectic! You get "border bloat" and way too many neighbours :p Just keep your economy strong and plan your advance.