I say you just have to go with the dice. Even if it is “game over”, there will always be another game.
I had a game a little while back where I, as Germany, was poised to take Moscow. In my first attack, the Russians managed to put units on Bryansk which blocked the majority of my units to the south. As a result, I barely matched the Russian defenders. I had 1 fighter and 1 bomber that could reach Moscow and support the battle. The Russian AA gun fires and gets a hit for BOTH planes! I couldn’t believe the luck. That battle was lost by the way and the Kremlin was saved.
Next round, I retook Bryansk and moved many more units up front, including planes. I moved in on Moscow and this time had 10 planes (2 bombers, 5 tacs and 3 fighters). With all of that to shoot at, the Russian AA gun didn’t get A SINGLE HIT. I overwhelmed the defenders. Moscow was captured, the Kremlin destroyed and Stalin was executed for War Crimes against the Fatherland.
What I’m getting at is that’s just the luck of the roll. You have to just go with it. I have defended with 3 submarines and gotten 3 hits. I have also defended with fighters and gotten all 5s and 6s. It’s frustrating but it’s still just a game. Take the loss, lick your wounds and come back to conquer the next time.