edwin states that ''you can never heal completely until you discover one thing''but does not tell Cole what the thing is.What do you think Edwin s referring to?

He doesn't say in the chapter. I think it has to do with perspective and attitude. I think it means Cole has to let go of his anger completely. 

I know I may not be completely right but, maybe Edwin is telling Cole he needs to discover a certain feeling in his body. I say this because he says, you can never HEAL until you discover one thing. Maybe that one thing is a certain feeling like learning to stop blaming others. Im not completely sure though !