The mentally ill may have shattered lives, but how that is different than the way sin distorts our ability to comprehend who we are as God's creatures is not clear.
Whatever it means for us to exist, we do so as creatures created, as the universe has been created, to glorify God.
I cannot imagine a more realistic faith than the Christian faith. At every turn, we are told we are death-determined creatures and that our lives, our all too brief lives, at the very least will be complex if not difficult.