Lying over and over again, getting caught over and over, and then having the AUDACITY to try to act like the past that you had no place in is worth discussing and excuses your actions. While you were cheating on her, Setsuko was finally confirming her feelings while DENYING a man's advances, one that she loved at one point.. While you had sex with a man you met only one time. And you weaponized her words to you as a defense as well, leading to her blaming herself for YOUR actions?!
The best you can do when you realize you fucked up is vomit a weak excuse like, "I don't know what I was doing?" You did.
"I felt so lonely... and I don't know why, but I felt so scared." So if Setsuko felt that last night when you didn't come home, she could have cheated on you too? You can go around and cheat when you feel lonely?
Deal with the consequences of your actions and hope that Setsuko, the wonderful woman that has been by your side this whole time, can somehow heal from your horrific actions. That damage cannot be undone, and the relationship will never be the same.