At first all he could hear was "I don't hate you", as his world slowly began to come back into focus. And then Remus was sat beside him, and it didn't look like he was going to jump up and leave any time soon, so Sirius could begin to breathe again, in a somewhat more normal manner.
He took in what Remus was saying, really took it in, and he was appalled - at himself. At what he'd almost done. At what might have happened. All because he was jealous of Severus. Why? He didn't know, couldn't say. This feeling sprang from nowhere, but it had eaten him up until he'd decided to play a little joke on him, scare him worse than he'd ever been scared before, teach him not to go where he didn't belong.
But he had clearly not thought through the ramifications of his actions, which had almost been fatal. And worse than Snape's death, which he could have lived with, was the knowledge of what that death would have done to Remus, and that was not worth considering, 'cause that thought was horrible indeed.
How could he be so stupid? Especially when he wasn't really a stupid person, at least he never thought so. But right now he felt like the stupidest wizard in the world.
At Remus' question, he studied his own hands, not daring to look at his friend. Finally he looked up, staring into those topaz eyes. "I don't know," he said honestly, "I wish I knew what to say, other than I'm sorry, and I'm stupid, and I was just jealous and I don't know what I was thinking." The word jealous just slipped out. It's how he felt but he didn't know why. He just knew something about Severus was driving him crazy when it came to Remus, and he wanted to hurt Snape. But not kill him, never kill him.
"I'll do anything you ask, Remy," Sirius said, laying his hand upon Remus' knee, fixing his gaze upon him. "Anything. I'll go down on my hands and knees and beg your forgiveness." And good to his word, he did so, there in the narrow confines of the quidditch bleachers, while keeping one hand resting upon Remus' knee. "Please please please forgive me," he whispered hoarsely.