Martin Heidegger considered his ‚Sein und Zeit‘ ultimately unsuccessful in its goal to think Being outside of its own forgottenness. Here we investigate Heidegger’s theory of forgetting and forgottenness in both ‚Sein und Zeit‘ itself and Heidegger’s later works on ancient Greek thought. This investigation is facilitated by the deep structural affinities of Marcel Proust’s treatment of forgetting and forgottenness through his concepts of ‚mémoire volontaire‘ and ‚involontaire‘ to the many difficulties that Heidegger encountered. With the help of Proust, then, we are able to not only better understand Heidegger’s own conception of forgetting but also the ways in which Heidegger’s difficulties with forgetting obstructed the project of ‚Sein und Zeit‘.