Sam feels like he never going to end his confession.
The more things he mentions, the more things come to his mind. He hears Dean saying that he never looked for him, mentioning being soulless, reminding him about Ruby.
He feels like he'll always let him down in the end. That no matter how much he want to do right thing, no matter how he tries to atone for his sins, he'll always be disappointment. It's not the demon blood that is tainting his soul, it's just pure him. Maybe he was always deceiving himself, trying to convince himself that the darkness inside of him comes from other place than his weak mind and heart.
He wants to say every single one of his sin in order, starting from the time he walked out on his family to start college, finishing at letting himself have life with Amelia while his brother was in Purgatory, but it all gets messed up in his head, sea of regret and ache flooding through his body.
In the end he just keeps telling "I'm sorry, I'm so sorry" but he knows that it's not enough,
Sam tries to keep himself focus, to think about absent God who's supposed to give him strength, even by pain and death, but his mind endlessly provides him with image of Dean's face, expression full of hatred and disgust,
"I'm sorry, Dean. It's going to be ok. I've got this." Sam whispers "This time it's really going to end."