There's a lot of negatives that could be said about Nanao, arrogant, insensitive, conceited, cruel, among others --- I'd say it boils down to immature and single-minded. What can you really expect from a teenager with that kind of high-stake goal?
Sawa isn't stupid, he knows pretty well what he's getting into (even though he's also very young when it all starts), he doesn't make even one demand on Nanao, not a single promise is broken. A lot of what's hurting him is also what's drawing him to Nanao in the first place. Even so, I still understand very well that he's disappointed and heartbroken by all of it. Calling it quits was the right thing to do, the way he did it though felt cruel as well.
Talk about flawed characters with unlikeable traits, but I still found them very relatable even in their respective cowardice. I thought the years they spent apart was a good amount of time to make it believable that they actually grew (up), changed, and built their own lives, and I was satisfied with the ending.
(although I would have also been fine with them not ending up together).
I found this to be pretty exceptional among the yaoi that I've read; no simple misunderstanding to clear up and no easy answers, people and their relationships are complicated, and love doesn't conquer all. There's a lot for a one volume manga, but I thought the artist conveyed it all really well. There is zero fluff and it stings a bit reading it but I enjoy it every time.