After the Kholi dance he finally let go of his inherent sadness after Arpita's death and thought of giving life (or shall i say Aarti ji as life) another chance so that he would smile again... maybe some day live again. He was slowly but surely moving towards the positive road... leaving his deep depression behind for his kids, his family and maybe Aartiji unconsciously...
He still was living in his Arpita's memories but slowly and surely coming out of the shell to live again. When the fire burnt his precious memories... even though for the sake of his kids and family and humanity but he had to chose Aarti. Though he lost his reason to live that day but still he went ahead and saved a life... the mother of his kids... maybe somewhere Aarti too... may be as a friend if not his wife.
He had slowly started enjoying Aarti ji's company, slight fights or her scolding him like the shop scene, her zid like in bangle scene, her persistence like mobile scene, basically all her antics when she was with him and making him come out of his loneliness... yes he had started liking her presence, he had finally found a friend in her... maybe more than a friend... he could communicate with her or maybe she knew how to make him communicate with her, even if it was via eyes... she could reason with him even if it was silent gestures...
Slowly but surely he thought maybe he also had a right to move ahead... he was taking baby steps in the direction of reviving his life of living again... with the hope deep inside his heart that she will guide him to light...
When she confessed her love but denied as a joke... he knew there was more to it... but he let it be... it disturbed him as he felt maybe he was betraying Arpita, his heart wasn't ready... so he pretended it was a joke but somewhere deep inside he knew it was more...
Now comes the bomb when i think he see Aartiji with Aman at 3 in the night coming back home (and from what I saw in the episode, the stupid guy might still be flirting at night 3 am with her) and maybe other close up scenes like the pictures suggest and boom Yash's world came crashing down.
Yash realized he did a blunder, he made the biggest mistake of trusting Aarti ji that indeed she would bring him out of that dark tunnel. He was jolted to the reality that it was no more than being the mother of his kids... that he was foolish to have assumed anything otherwise. Realization drowned on him that he betrayed his Arpita and her memories, that he tried to live life again but it was all a sin, a crime maybe he ain't worthy of it, maybe he never deserved a second chance to live.
He thinks himself to be the biggest fool to have ever believed Aarti ji's unsaid emotions... that is why he said "I have no right to punish you Aartiji so I will punish myself". He will torment himself for trusting her, for trying to come out of the darkness, for trying to live again. He will suffer till he dies as he never had any right to get a second chance at life.