You don't know the whole story.
It's none of your business.
Everything must have worked out for the best.
You shouldn't even know about it anyway.
Everyone seems happy enough.
It ended years ago.
There's nothing you can do.
Korra repeated mantras of logic in her head as she pulled streams of water up out of the bay around Air Temple Island and whipped them around herself. The important thing to focus on now was the championship tournament tomorrow, not her mentor's love life... past, long-since-over love life. It didn't involve her at all; there was no practical point to dwelling on it, so, logically, the only thing to do was to forget about it.
As the Avatar spun around, her eyes happened to pass over the clearing where she often tried Airbending when she was alone. A newspaper with Tahno's face grinning smugly at her still hung from the clothesline. She would be practical. She would be logical. But, oh, how she wished she had a picture of Pema to use for Firebending target practice!
Korra sent the water over the cliff, back to its home, with a sigh of defeat. All the logic in the world couldn't erase her anger at Pema, however unjust she knew it (most likely) was. When she remembered the confident, satisfied way the woman bragged about stealing her husband from his first love...
Disillusionment stung. Korra had pictured Pema's rival very clearly in her head: a shallow, brainless beauty, a powerful Bender dripping with charisma and probably wealth but with no heart, no true love for the Avatar's son beyond the fame and status marrying him would bring her, who had easily forgotten about him and moved on to live happily ever after with some wealthy noble who could satisfy her desires just as well. In reality, she had been a close childhood friend; a partner in his mission to protect the city left under their charge from its enemies; a strong, fearless, noble, heroic warrior who inspired awe and respect in everyone she met, friend and foe alike; a woman whose affection couldn't possibly be earned easily, but she had nonetheless bestowed it on him... and who had never married. True, they were very different; even their elements were opposite. But as they say, "Strange extremes meet in love's pathway."*
You have no right to be angry at Pema, Korra told herself again. You have no reason to be angry at Pema. You don't know the whole story... Back to square one.
Korra left the cliff and headed for the woods, hoping a walk would shake off the bitterness that had taken over her. It was getting annoying, actually; why was she really so angry at Pema? She'd always liked her whenever she visited the South Pole, they had always gotten along, and she and Tenzin always seemed happy. It didn't make sense; her mind must be hiding something from her again, just like she had successfully hidden her fear of Amon from herself until a few weeks ago.
Korra slowed her pace, took a deep breath, and tried to search for the truth in her mind, like how she'd been taught to search for moisture in the air when there was no other source of water to bend.
Maybe you're really mad at her over something else, thought Korra. Like the horrible advice she gave you about Mako. Korra shuddered as she recalled that moment when she'd been too high on their first tournament victory to stop herself from doing the... second stupidest thing she had ever done in her life. But she had never blamed Pema for that; she had been too furious at herself and then at Mako to spare any fury for anyone else.
No, that wasn't it. So what was? The sound of a splash interrupted her meditation. Korra looked down to find she'd stepped on the edge of a large puddle of melting snow. As she took a step back and Waterbended her boots dry, the surface of the water calmed, and her reflection came into focus. She stared down at her reflection for a moment, with a look of such intense concentration, you would think she had never seen her own face before. How long she stood there, still and silent, looking into her own eyes, she didn't know, but it was long enough to make her realize the shameful truth.
You're not angry at Pema. You're angry at yourself.
Even though telling Mako they were made for each other had painfully backfired, Korra had held on to the fantasy that it was true, that they belonged together even if he didn't realize it yet, that it was her duty to protect him from ending up with the wrong woman, that stealing him from Asami was what was best for him. Pema did it, and it worked out for the best for her and Tenzin, Korra had told herself. She was in the same position, and it would work out best for her and Mako, too, if she had the courage and determination to make it happen. If she cared about Mako as much as Pema cared about Tenzin, it would be selfish and wrong to leave him to someone who didn't deserve him.
Now, that fantasy had evaporated along with her vicarious joy over Pema's victory. A woman's heart had been broken, a friendship had been ruined. She didn't know for sure that Tenzin's wife had been unquestionably wrong to steal him away, but Korra could no longer believe that she had been unquestionably right. Which meant she could no longer tell herself it was unquestionably right to spoil Mako's happiness with Asami.
Wake up! It wasn't selflessness, compassion, or concern that made you say 'When you're with her, you're thinking about me.' It was pride.
Everything she had said and done to Mako between hearing Pema's story and the semi-final match had been motivated by selfish pride. Her only concern had been her own happiness, not his; her doubt over whether Tenzin's first love deserved to lose him made her quite undoubtedly certain of this.
Korra had reached the side of the island facing the Pro-Bending arena, where the two brothers who (a few days ago, at least) had every right to wish they had never met her were probably training right now. She stared at the view and thought of how she had mistreated them throughout the tournament, putting one unfairly on the spot, and leading the other on before kissing his brother. She thought of Lin Bei Fong and how obvious it was that she hadn't "moved past it" as thoroughly as Tenzin had. She thought of Tenzin standing by her side determined to protect her from any attack and had to wonder if he had "moved past it" as thoroughly as he claimed. She thought of Pema confessing to breaking them up without a single sign of regret.
Well, if she was going to be completely reasonable, she didn't know the whole story, but she knew enough to concede that she could no longer use Pema's actions to justify her own. "Those who don't learn from the past are doomed to repeat it," the sages taught her about her duties as the Avatar. As Korra stared across the bay, she silently vowed she would not cause whatever painful past existed for Lin Bei Fong and Tenzin to repeat now for Mako and Asami. She would never intentionally be the cause of unnecessary heartbreak. She would never try to usurp affections that already belonged to someone else. She would never be that type of girl.
Korra turned her back on the sight of the brothers' home and headed towards the house, feeling lighter and freer inside than she had for days.
*Quote from one of my favorite novels, The Scarlet Pimpernel (the first super hero) by Baroness Emmuska Orczy, whose conflict between the French aristocrats and the revolutionaries is very similar to the conflict in Avatar: The Legend of Korra (I don't pretend to know how historically accurate this interpretation is...).