1) Namgung returns to find Mi Do married to Tae Seung
Namgung goes abroad to rebuild his career and finally escape a relationship that has been hurting him. Years later, he returns to find Mi Do married to Tae Seung, with a child together. Mi Do tells him she will always be grateful for everything he did for her.
The ultimate insult? Namgung gets gratitude instead of the love he sacrificed so much to protect.

2) Mi Do marries Namgung after all, but still loves Tae Seung

This might be even more consistent with the character you've described. Mi Do chooses Namgung because she cannot abandon him, because of their history, or because she feels indebted to him. Everyone celebrates their reunion, and Namgung believes he has finally been chosen.

But the audience knows that Mi Do has never honestly confronted her feelings for Tae Seung. Namgung gets the relationship he wanted, but not necessarily the love he deserves.

3) Namgung sacrifices everything, then graciously gives Mi Do his blessing

Namgung discovers Mi Do's feelings for Tae Seung, steps aside without fighting for himself, and tells them he hopes they will be happy. Mi Do cries, Tae Seung embraces her, and Namgung walks away alone.

The drama frames his suffering as proof of his noble love. Nobody is held accountable, and his emotional devastation becomes a convenient way to give the other couple a beautiful ending.

4) Mi Do chooses Namgung at the last minute, and we're expected to forgive everything

After spending the drama running to Tae Seung, Mi Do suddenly realizes Namgung is the one she truly loves. One emotional speech, a few tears, and perhaps a flashback montage later, all her contradictions are swept aside.

Tae Seung accepts the loss, Su A moves on, and Namgung welcomes Mi Do back. The writer calls it a happy ending, while the audience is left wondering whether Mi Do has understood herself at all.

5) The double betrayal: Mi Do chooses Tae Seung, but Namgung still saves her

Mi Do finally admits she loves Tae Seung and leaves Namgung. Yet when she faces a crisis, she turns to Namgung again because he is still her safest place. He helps her, asks for nothing, and lets her go back to Tae Seung.

This would crystallize the most frustrating aspect of their relationship: Mi Do gets to pursue the man she wants while continuing to rely on the man she cannot bear to lose.