What I believe is missing is the fact that to Prime, he feels like he's facing an onslaught. One after another, the Decepticons keep rolling in. To him, the situation seemed only to be escalating.

Put yourself in his shoes. That many Decepticons keep knocking on your door, you're bound to be agitated, frustrated and worried. Who will be next? Hence why he shows the signs of cracking under pressure. Even worse, he's not even a fully fledged warrior. He's had training and then spent cycles as a space bridge repairbot before facing down Decepticon after Decepticon. If I were him, my pants would be rather brown and not very aromatically pleasing.

All around him he sees, quite fairly, is a lack of discipline. When times are tough he needs to run a tighter ship and his inability to do so is really getting the better of him.