NOTES ON THE ORIGIN OF THE GENERAL THEORY OF RELATIVITY

"These were errors of thought which cost me two years of excessively hard work, until I recognized them as such at the end of 1915, and after having ruefully returned to the Riemannian curvature, succeeded in linking the theory with the facts of astronomical experience. "In light of the knowledge attained, the happy achievement seems almost a matter of course, and any intelligent student can grasp it without too much trouble. But the years of anxious searching in the dark, with their intense longing, their alternations of confidence and exhaustion, and the final emergence into the light--only those those who have experienced it can understand that." p. 289-290, "Notes on the Origin of the General Theory of Relativity," IDEAS AND OPINIONS by Albert Einstein (1954, 1982)