Remus didn't say anything. He couldn't let himself, because he knew that if he opened his mouth, he'd forgive Sirius...just like he always did. Only this time, he knew it would only be words. He never wanted to hurt Sirius, and he did still love him, but he couldn't forgive him for this right now. It wasn't fair to either of them. To himself because it was giving in to weakness and setting a precedent that this behaviour was easily forgiveable. And to Sirius because he didn't forgive him and if he said so he'd only grow to resent him.
He continued to look steadfastly at the floor, waiting until he heard the door close before allowing himself to break down. That had been, by far, the hardest thing he'd ever had to do. He felt like he'd ripped out a part of himself, sending Sirius away, and he didn't know what he was going to do now. He hadn't felt so alone since the first full moon, when his mother wouldn't touch him and his father had locked and warded his cage. He'd known in that moment that things would never be the same, and he felt that sickening certainty again when Sirius had left.