I don’t know if Einstein did his quote in German or English. My translation of the German text would be:
“You image that I look back with quiet satisfaction on my life’s work. But, it is quite different considering from nearness. There is no single concept from which I would be confident that it will be stand and I feel unsure if I was ever on the right path.”
Not only Einstein was ever on the right path, but, I am sure that the whole modern physics was never on the right way since Max Planck’s introduction of the action quantum, 1900.
The Planck constant h is not an action quantum, but h is a Lagrange multiplier which appears at the variation because of the subsidiary conditions of conserved particle numbers.