Well it's for that reason that I think it's easiest to learn the game using nations that have strong PD. So, you raise the defences on provinces that you aren't keeping active forces. The AI likes to minimize the forces that it needs to take a province, sometimes attacking with 8-9 soldiers in a force (not strongly blessed sacreds, like if it were a human
), so you just need your borders protected enough to stop that. However, the sooner you train yourself to manipulate smaller mobile forces, the better off you will be. Not just because mobility will win you a war that brute force can't, but because if you stick yourself only taking 1 province per turn, you won't grow very quickly.
And likewise, as you progress in the game, if you find that even a massive amount of your forces gets crushed, then you are not keeping up with the arms race. Start developing your use of mid-game magics and summons more deeply. Some of them aren't that big a deal, or worth spending gems on. But some of them can easily mean the difference between an embarassing loss, and a heroic triumph.
Also, with most nations, you'll end up looking at your excess castles not as troop factories, but as mage training facilities. Even if you are using a nation that has fairly weak mages in its noncapital castles, look at the limited paths you are getting, then explore the spell lists. You may find that there are 1-2 buffs that you can cast like crazy with enough cheap little mages, that all of your mediocre mundane infantry suddenly performs very very well in combat.