Heidegger believed we are truly ourselves when we are "authentic"; when we inhabit all aspects of our thrownness and our ability to choose. However, we are constantly pushed into being inauthentic by falling in with the crowd (Heidegger describes society and the influence of other people as the "They"). Our individuality gets curtailed when we are immersed in the They. How do we recover our authenticity and bring us back to ourselves? We need something to hold onto that can only be our own, thus preventing us falling into the They. Heidegger chose our being-towards-death; our death, when it comes, will be ours alone. No one can die another person's death. Therefore, thinking about what we lose in our death reflects back to us what makes us individuals during life.
We can also get lost in the crowd of our Parkinson's symptoms, until we lose sight of ourselves. However, it follows from Heidegger's argument that ironically it is our unique experience of these symptoms (no one can live them for us) that brings us back to all aspects of our thrownness and who we are. We can be authentically ourselves even within Parkinson's.