He's shaking, definitely doesn't believe his words.

"I- I'm sorry. I thought I could," he ducks lower (making himself a smaller target). "I- I can't- my focuse- I'm sorry. I'm supposed to be- learning to control-"

"And you are learning. You're doing very well, and control is something that will come with time." Castiel looks on as if recalling something from far in his past. "In Heaven, when there were still fledglings running about, it took them years to learn how to control their grace. I don't expect it will take you nearly as long, considering you've already had time to grow accustomed to your power."

He licks his lips, nods (with a hint of the same disbelief that's coloured many of their interactions). "I- as you say, sir," he agrees. He doesn't make a move to try again though.

"Would you like to be done for the night?"

He's still for a moment, still looking for a trap. "Please?" he requests after a long minute. "I- If you don't mind?"

"Of course not. You are obviously exhausted. It was wrong of me to push you in such a vulnerable state. We can continue this tomorrow." Castiel looks concerned as he stands up and watches Nova leave.

"I- yes sir, thank you sir."

He stumbles a little as he leaves, but doesn't actually fall over. He can be heard making his way back to his room, where he curls up in the space he made at the head of the bed (it feels safe).

Castiel stands in the living room, monitoring Nova's soul long enough to ensure that he won't try to hurt himself again. 

He doesn't. But that doesn't mean he won't in the future, just that being told not to is still fresh in his mind.

The next few days pass uneventfully, the only change being the subtle improvements in Nova's control over his grace and abilities.

Nova seems to settle somewhat as he realises he is making progress, even if by and large he is still somewhat unsettled by the strange change to his living arrangements, and perplexed by his new guardian. If there are any more incidents of him harming himself, they're cleverly hidden as accidents.

Castiel stayed close, never leaving for more than a few hours at a time to buy food or other resources for Nova. About a week later, Castiel receives a summons to heaven to give a report on the Nephilim's progress.

He isn't doing badly. His control is improving, and while he's still timid and unsure, he seems a little less convinced that Castiel is going to hurt him for making a mistake.