I see this in students all the time, and I always give the same advice: talk to somebody. Find a friend who would be willing to listen and challenge you on every point. Sit down with a piece of paper, and try to tell her/him the whole story, explain the theorem you are trying to prove, examples, counter-examples, etc. Even if your friend can't help you formalize the ideas, the act of explaining itself might prove very useful to clarify the matter. Good luck!