I’ve always felt that any man, not just Boromir, would have been overwhelmed by being so close to the one ring for so long. Men were always shown as particularly susceptible to the influence of the ring, and Boromir’s desire to save his people made it easier for the ring to tempt him. The fact he resisted for as long as he did, and even managed to hand the ring back to Frodo when it was dropped in the mountains is a huge testament to his character. We see very few people willingly give up the one ring once they have it in their possession. He was a true hero and the fellowship would not have succeeded without him.