The ability to build a "Fort" might help.
Fort:
1 Turn
100 Gold
20 Defense
20 Supply
No Towers
0 Administration
Does Not Allow National Troop Recruitment
Upgrades to National Castle
I completely agree, though. It's so hard to defend your territory. Even if you split your army in 20 to cover all border provinces (and thus allow inevitable defeat-in-detail), you are still vulnerable to Ghost Riders or Hordes from Hell throughout the interior.
Of course, making Province Defense substantially stronger (and also giving it better units when a province has a fort, castle, temple, or lab) will go a long way to swinging the balance around. The enemy can't MIRV their midsize army against your 5 border provinces if each has PD 16, a fort, and a temple if those investments give them a real defensive force. And with an "Attack Army" or "Defend Provinces Within Range" command for a defending army, the invaders can't simply slip by because you mispredicted their target. Realistically, the US Military units in Utah wouldn't let Canada take Washington without coming to help, simply because the invasion was not listed in the month's "List of things to do". Washington units might not bother helping Utah, though, because there's really not much there
Cheap forts, stronger province defense that improves with buildings, and a "Defend Provinces Within Range" command would all be very nice features for Dominions III. Defense can be almost impossible without them, often making it an "Attacker = Winner" type system.