[Alphaelin]

I have to say that I don't think past traumatic events were ever intended to be a motivation for any of these characters. Though JRRT could very well have seen first-hand the effects of shellshock during his service in WWI, he did not write Lord of the Rings to create a psychological study. His passion was epic literature, and that is what he wanted to create. Characters in the epics I've read (LOTR, The Iliad, The Odyssey) tend not to be written with a lot of internal angst. Instead they are intended to show their response to a given quest or situation: destroying an evil ring, defending one's city, winning back a prized possession, getting back home again.

While Boromir would recognize the disastrous nature of the loss of Gandalf in Moria, he should be one of the least affected members of the Fellowship. Not only has he seen violent death before, it is his job both to kill and to send other men to their deaths. In order to do his job, he would have had to learn to cope with grief and guilt long before. While it might be argued that Gandalf's death might be the last straw for a mind that has seen years of death and destruction, the ring starts working on him before he even leaves Rivendell.

His vulerability to the Ring is based in his personality flaw of pride more than a traumatized past. The Ring, being completely evil, is able to capitalize on Bo's pride, and even on his determination to defend his city. Boromir never quite sees the big picture: Middle-Earth must be saved even if Gondor falls. For him, Gondor must be protected even if the rest of Middle-Earth falls. Thus he is able to justify attacking Frodo by telling himself that he needs the Ring to protect Gondor.