Heard he wrote a bunch of rambling thoughts near the end of his life and doubted his own theory of relativity...it was in Einstein: His Life and Universe by Walter Isaacson
He opposed Quantum Mechanics from the beginning until death. He did it because its inherent non-determinism ("God does not gamble"). In his opinion, the cause of the non-determinism of the QM is that "it is not ready".
However, he worked on the further development of the GR until death. A possible direction was to describe also the electromagnetism as spacetime curvature. This makes highly unlikely that he had doubted it.