It was inevitable they would grow more distant over the decades. She had her family and he had his career protecting Republic City. She always tried to make it for visits at least once a year. The unbreakable connection between them that he so fervently believed in was still there, but reason told him their chance for a future was long gone. He had been on a few dates, but comparisons to her always popped up in his head. In the end, he decided it was not worth the effort. He took pride in his awards and promotions from his career, but the more he achieved, the hollower they felt. Time and loneliness made him tired and bitter.
The two worlds mourned together. People and spirits made the pilgrimage to the South Pole nearby the Southern Portal where she requested to be buried. Korra knew her time was coming, she was felt weaker as the days passed. She had lived only a few years longer than Avatar Aang, but her battle with Unalaq decades ago finally took its toll. Having Raava ripped out of her that first time and using her cosmic energy to battle the Dark Avatar drained much of her life energy.
She died too soon and it was his fault. If he had kept Unalaq away from the portal a few minutes longer, Korra could have locked Vaatu away before the Dark Avatar was formed. He didn't care if the two worlds remained separate.
After the funeral, only her closest friends and family were at the compound where Korra grew up. Bolin told stories of their past adventures and everyone laughed and cried together. Her husband took a seat next to him and handed Mako a letter. "Here, she wanted to have this." It was still unsealed. Even in the end, her husband was still the better man. "I'll leave you alone to read it."
The lights danced in the night sky, just as beautiful as the first time he saw them.
"Mako, the days have never seemed longer, but I know my time is coming to an end. My end is only just the beginning though. The years are just being given to another Avatar. You were my first love and even though we couldn't be together, you are still family to me so please watch over mines. I never forgot how much I loved you and I will love you always, Korra."
He never read that letter again, but saved it in an old album on the same page where he kept the photo of the two of them at the Southern Glacier Festival so many years ago.