Contract your front further back so it covers less ground, and/or stretch your line thinner over the same amount of ground.
You've done a good job of keeping at least two units on your entire line this war, but there's been places where I've had little choice but to rely on single units fairly frequently, and that's probably been why you managed to push salients through my lines a few times at great cost to me. As long as they're fairly defensively-strong units in defensive postures on defensible terrain, it's a reasonable gamble. If you can't be sure you can put them in that sort of a position, though, contracting your lines is likely safer.
In a pinch, you can have a thin portion fall back so the enemy immediately bring as much force to bear on your lines b/c they need to advance to engage them. It's not ideal, but it's better than standing firm and getting enveloped if that's the other option. It's something I've had to do several times in several places this game, especially since when we started fighting I had an unpleasant percentage of militia troops on the line. Basically, buy time until you can reinforce, and plunder a city for recruits so you can reinforce. Just, uh, make sure you don't do that somewhere that doesn't have the logistics to ship them out immediately. Last game I drafted 34k recruits in a single turn when things were starting to get desperate, but did it somewhere with swamped logistics so I was never able to ship more than 3-4k per turn to my SHQ. I think there were still 5-10k recruits waiting to muster in that city when the game ended...