Now that I carefully think about what they said in the end, I deduced that they did probably live happily ever after. I think that the final thoughts they have about someone more deserving are themselves in the future.

He mentions, "One day a man much more mature than I am should take care of Aikawa."

She mentions, "One day he will find a cheerful girl much more suited for him than I am."

That mature person is his future self. He is only a high school student and has a long road ahead of him.

Aikawa too is still only in her infancy, and still has the potential (as we see her grow throughout the story), to become a "cheerful girl."

The whole, "the two of us must part," is used to speak about their present "immature" selves, that they will leave behind once they mature.

So until that day comes when they grow up, and have to start thinking about things like starting a career, finding a house and eventually raising a family (and in their case finding Aikawa's child), they are going to enjoy themselves. Because, as everyone knows, you are a high school student only once, and you live only once.

WOW, very well done mangaka.

I think that Diasuke starts out just deeply concerned with Aikawa. She's been through so much pain that he can't even fathom and so he wants to protect her like a wounded bird. This sense of duty made him actually feel... like his life had purpose and that sense of purpose makes him fall for her.

I mean, he starts out saying that he'd have any girl but the bitter fake or a virgin... I think the complexity of her character baffled him, the first interesting person in their small town and a person only he can be close to and help.