Hollywood, your lucky Apple didnt see this tongue
Yes Fredo was truely sorry for what he did, and he would have lived with that feeling for a year, maybe 2.But something would have come up again, and he would betrey Mike again, intentional or not.Fredo could never accept his place UNDER his YOUNGER brother.What he said in the boathouse sealed his fate. "I'm your older brother Mike, and I was stepped over! ...That aint the way I wanted it!" Meaning he was always going to try to be the Don, until he was dead.
Sonny from A Bronx Tale said it best, "Fear lasts longer than love" In the same way, greed lasts longer than sorrow.Fredo's feelings of guilt for hurting Mike would have lasted until another offer was presented to him. He would have never stopped.

If winners never lose, well, then a loser sure can sing the blues.