The quote you mentioned from Albert Einstein's private letter is indeed found in his correspondence with Michele Besso, a close friend and collaborator. In the letter dated March 15, 1954, Einstein wrote, "I consider it quite possible that physics cannot be based on the field concept, i.e., continuous structures." This letter is referenced in the book "Subtle is the Lord: The Science and the Life of Albert Einstein" by Abraham Pais, a biographical work on Einstein.
It's important to note that the letter represents Einstein's personal thoughts and reflections at that particular time. Einstein was known for his willingness to question and reevaluate scientific concepts, and he continuously engaged in discussions and debates with his peers. However, it's crucial to approach his statements in private correspondence with caution and avoid drawing definitive conclusions about his entire scientific worldview based on isolated remarks.
To gain a comprehensive understanding of Einstein's scientific contributions and views, it is advisable to study his published scientific papers, essays, and public statements, as they provide a more reliable and thorough representation of his ideas.