The feeling that there is something wrong with you in particular that makes things impossible while other people succeed is a common one, and Sans is self-aware enough to know that at least part of it has to be an illusion. He's not special.

So... by that same token, where does he get off feeling and acting like this while other people carry on? He raises one of his hands and rubs his face with it, trying to sort out his thoughts.

"I wouldn't... bro." His mind races trying to think of how to deny that. "I would never... do that to you. I don't even..."

His other three hands make vague grasping gestures at the air, like he's trying to pluck out the right words.

"I'm glad I'm still here. I'm glad... that I asked the RM to watch out for me. It was just... a lot to deal with. And when you were the one telling me I should quit..."