This is one of my favorite mangas. I've been looking everywhere for it. Of course it is not fair for Tsunayoshi's girlfriend but they are both mistaken. Tsunayoshi for trying to run away from his love of Misao and the girlfriend for not stepping back when she realized that he is in love with Misao. Of course, it is a heart-breaking situation, but love is something to be thought of maturely. It is something which should be mutual, hence no matter how much she loved the other guy, how much she needed him, she should have just stepped away and let him consume his happiness. If you don't reach a phase where you genuinely think of the happiness of the other person, even - and especially when it is realized without you -, then you don't know what love is. That would be the illusion of love, the crystallization of our weaknesses and inability to stand on our own feet.
Aniki wa Tsunayoshi