Remember, he keeps saying that his relationship with Swara is based on total trust. I think he took it for granted that Swara would trust him no matter what because he himself trusts Swara no matter what. That is his innocence that makes him so sure that Swara's trust in him is absolute. And in my opinion, he was justified when he expected Swara's trust to be unwavering. So, when she had doubts about him, he felt betrayed. He was genuinely very hurt. That innocence was reflected on his face in the scene. As you said, like a child, he said he would never forgive her. When a child is hurt, he seldom listens to explanations. He is in no mood to forgive because his pain is pure and innocent that words may not have the power to lessen that pain. Same with Sanskaar, his hurt was so pure that he doesn't want to listen or forgive. Its because his pain is so excruciating that he doesn't want to risk Swara hurting him again. He actually cries. It shows how important trust is for him. Sanskaar always has the best lines and Varun delivers it perfectly.
It was a sweet scene. Varun's acting was so good. Loved the scene.😳 I especially liked Swara's reaction to Sanskaar. She has been taking Sanskaar for granted. It's time she realized how important he is for her.😳