This is a beautiful and sad love story.
The ML puts his love ahead of his own future.
Through a magical watch he is given the chance to change the future to help the FL fulfill her destiny.
In the times they are together as a couple their love shines out.

The time travel has unexpected consequences and he continues to love and support her from a distance. His rapid ageing is done really well.

I kept wanting a fairy tale ending and we are offered this but it is not the truth. At the end there is a sad redemption and throughout the story we are given glimpses via the diary he keeps.