You're Dazai.

It was at a party. You saw him in the crowd; Nakahara Chuuya. He was casually dancing with a woman while smiling happily. He didn't even notice you presence there. But what could you do? You broke up with him because you were very indecisive at the time. It was your decision, and what a bitter decision it was. You continued looking at him from afar, a spark of jealousy slowly rising in you.