The awareness of the innevitability of death can actually be freeing and help give more depth to life. For instance Maranasati practices in most branches of Buddhism or the "Memento Mori" of the romans and middle age christians. I am actually even convinced that death, even with all the pain and despair it brings, is necessary for beauty and mystery. And I say this as someone who had several close relatives dying recently and went through grief and accompanying the grief of others.

Midlife crisis might actually be an example of that: "why am I waste my time? It is limited, I need to make something meaningful" would likely not be a common thought if we were immortal. You cannot waste what is in infinite supply.

(Note that this is a theme one finds in Buddhist cosmology: birth in the human realm is seen as better than birth in the heavenly realms, where life is so long it is essentially infinite, because it is human mortality that helps getting the drive to study the Buddhas teachings to end the cycle of rebirth. Very refreshing idea if compared to classical conceptions of the Christian paradise)