At first I thought this was a poor convoluted mess of an alien invasion, considering it to be very poor in its worldbuilding and timing for the stakes I assumed there were, however I was pleasantly surprised midway through.
This is a beautiful metaphor to accepting your feelings and surpassing regretfulness, to move forward, in its own meta way.
I feel the last explanation (About Ashita writing all from the beginning) was redundant, since the message of this manga was perfect at its subtlety, but it makes it no less a good ending.
It wasn't perfect, there were way too many characters, and the whole betrayal part was mediocre at best, but the emotinal charge the main duo and eventual maturity of them makes it worth reading to the end.
7/10