Boromir was the only member of the Fellowship who tried to take the ring. He was vain and entitled, believing that he alone, of the Fellowship, was worthy of the ring. He was a thief and a traitor.
Boromir was a Lord of Gondor, and he wanted everyone to know it. “His garments were rich, his cloak was lined with fur, and he had a collar of silver in which a single white stone was set.”
Boromir did not redeem himself. He failed to protect Merry and Pippin from the orcs, who wouldn’t have found the hobbits wandering alone if it weren’t for Boromir’s actions in the first place.
Boromir would not have felt remorse or apologised if he had succeeded in taking the ring; he only did because he was caught. His image was so important to him that his “heroic” death was staged to create sympathy and goodwill so that he would not be remembered through the ages as a thief and a traitor.
Boromir got what he deserved.
I don’t know the lore too well, but long life and invisibility that the ring bestows on mortals ain’t going win any war.
That ring’s primary domain is domination, with other extra powers given depending on person. In the hands of someone like Gandalf or Galadriel the war would most likely have turned before the ring inevitably betrays them, or they themselves becomes as bad as Sauron.