As soon as he had all his magic back, he was figuring out a way to curse the words 'I'm sorry' from Remus's vocabulary.
"And what would've happened if James hadn't found you?" Sirius asked, voice falling flat. "You could have bled all over the rocks and died. Even if I couldn't have prevented it, I should have told James to go after you rather than leave it to chance. It doesn't matter what you expect because you never expect much of anything from anyone no matter what it is. But I'm your friend and I should have been there."
That's why he felt like a royal arse. He knew there was nothing he could have done to prevent the breaking out and the jumping off cliffs– but he could have put his pride aside and been there to help Remus. And, you know, not fail as a friend.