"The question of the Ring is something that has been nagging Boromir since the journey began. Because he’s a human in a land of men, he is more susceptible to its power and more likely to be tempted. This is a thing, that throughout the journey he is always trying to keep down. And keep back this temptation, this sort of need, to hold the ring. To just have it in his presence. At the end , when he sees Frodo, in one of his weak moments, it finally overcomes him. It’s like a drug, eating away, he’s overcome with it at that point. Then he’s totally sorry, he’s devastated by that and in a way, he feels he can’t go on anymore. He feels he can’t be whole again. He’s let everybody down. So he’s made a journey and he’s a better person for it. Because he’s come to see that there is actually more to his existence in Middle-earth then he originally imagined.
He originally imagined it military and fighting. Gondor keeping the enemy at bay. He wants to just use the Ring to fight the enemy. This is foolishness, let’s use it. He becomes more world-wise and open. He says to Aragorn at one point, ‘I didn’t realize what I know now.’ But by then, it was too late.”
— Sean Bean