[Keeper of Dol Guldur]

Actually, yeah it is pretty obvious that everything fell apart after Gandalf was lost, not just in his leadership (Aragorn did a pretty decent job of it) but because of the trauma itself. Here was somebody that Aragorn and Legolas knew to be an ongoing entity, and who to the rest seemed immortal and unquenchable in that regard, falling. And they all had known him for their entire lives.

After Gandalf's fall, Aragorn grew testy, Boromir lost confidence in his own strength and Gimli grew louder if anything. Other than that, Legolas stopped noticing nature to some degree (he didn't stop to listen to stones on the east side of Moria, and only the Mallorns really peaked his interest), Merry and Pippin stopped being jovial and shut up, and Sam and Frodo were even more silent.

But back to Boromir, it does seem his assault on Frodo was a bit unlike himself. Obviously it's supposed to be like that, with his mind being drawn by the ring. But maybe that was the ring's effect, after all a man who had fought in wars and seen many good men fall in battle shouldn't have broken down that much, and for somebody who didn't have much hope to begin with for the quest, how could it have worked now without Gandalf's leadership? While Lothlorien offered a respite from their worries about such things for a while, once they hit the river, Boromir's mentality deteriorated with every mile the river took them.