He's aware of his dysfunctional relationship with Kotone - he's not blind to her faults. He knows exactly what kind of person she is deep down, even if he might not be aware of her more sinister side. He knows that she's a difficult person to get along with, and THAT is the difference between other shoujo males and Sasamoto - he chose to fulfill his duty of staying with his clingy, obsessive, weak friend over his own love for his girlfriend... and he knows it very well. He knows that his thing with Kotone will inevitably get in the way of his and Hiyori's happiness, and coupled with his belief that Hiyori could deserve better, he lets her go.
It's easy to say, KOTONE IS A BITCH HE SHULD LET HER G0 but think about it from Sasamoto's perspective. Is it that easy to sever a relationship with a woman that came with more baggages than you can shake at? Is it that easy to simply walk away from a history that bound them both? No, it's not.
That is why Sasamoto "chose" Kotone. He doesn't jump to and from both women; he's able to look at the bigger picture and make a decision to fulfill the obligation he made with his longtime friend and accepts the burden of caring for her when no one would. Yes, it's a burden - and he knows it fully well.
But above all, if there is one thing you cannot accuse Sasamoto of, that would be being ignorant and indecisive. As disagreeable some might find his break-up decision to be, it was glaringly obvious he did it, not because he "couldn't make up his mind", but because he rationalized it to be the most sensible out of the current circumstances, for what he thinks were to the benefits of both women.
Nowhere had he placed his own interests on top of it. Sasamoto knew exactly what he let go when he separated from Hiyori. This is why he cried, after all.
The comment below where the poster refers to the manga as "stupidly unrealistic" is pretty amazing as Sasamoto's decision and Hiyori's inability to forget Sasamoto are reflections of ordinary occurrences in real life, even if a bit exaggerated but by no means as terrible as he/she makes it out to be. It's frustrating, slow and unfortunate - does that sound familiar? Yes! If you're looking for something cheesy, straight-forward and mindless, go look for Shinjo Mayu instead.
For those who want to know how it goes and whether you want to ragequit after Kotone's antics, I've read up to the latest chapters, and mark my words...
I don't mean for this to turn into a "Sasamoto vs Yuuma" thing with this remark, but I'd just like to say that Sasamoto is by no means as stupid as some of the posters are making him out to be, and Yuuma is nowhere near as mature as the posters are claiming him to be too.