The Sixth Station

It all started when he woke up to find himself inside a flying old train.

"What in the world...?" He said to himself as he looked to his window, seeing nothing but an endless sight of clouds and nothingness. No signs of any land or water.

Was he... really dead?

"Do not worry, my dear. You are not dead, but your soul is not on Earth anymore." A luminous figure floated towards him and sat beside him like a passenger on the train.

"W-who are you? Where am I?"

The goddess-like figure sighed, "You and Thana have been going through this cycle. Both of you cannot keep going on like this any longer. It's time to break that cycle, Tristan."
Finally, she looked at him and smiled, the warmth reaching into his soul. "I am Arianrhod, Goddess of Rebirth and the weaving of cosmic time and fate. I am here to help you clear and balance your karma."

Tristan and Thana are not ordinary soulmates. They are "Karmic Soulmates", their tie is bound to a certain karma that needs to be resolved. With every life time they fail to clear that karma, they will be reincarnated again and again until the balance is established, and finally ascend to pass on. Until, the goddess decided not to summon another life time for the two unique souls. Instead, she gave a different mission. He will be sent back to his previous life times to figure out what it is that is keeping them bound to a vicious unfortunate cycle and somehow change the outcome so that it will reflect to the other life times. However, it comes with a cost. Whenever he is to fail in his attempts, he will be sent back to the sixth station (the train) as he loses fragments of memories about her and their situation, making his mission more challenging to complete.

He will have to endure replaying the mess and her death over and over again. It might drive him to the edge of his sanity, but only he could stop the curse.

So what is the right way to solve this?

Is he supposed to save her? or save himself?