Remus wanted to be able to hold his hand and reassure him that everything would be all right, but he felt he had to keep his distance. Severus indicated he needed quiet and to focus, so he just stayed there, close enough for Sirius to be able to sense his presence but that was it.
If this didn't work, he wasn't sure he'd be able to keep it together anymore. Remus had been trying to be brave all this time, but he was growing weaker. With the Full approaching, he felt even less together, and he was afraid of how that would affect his resolve. He was afraid of how that would affect Sirius. For now, he had to put all his hope and faith in this working. Otherwise, he didn't know what he was going to do.