The Mortal Immortal by Mary Shelley

A man drinks an alchemical potion that grants him immortality, but over time, he discovers the endless pain of watching others age and die. His youthful appearance becomes a curse as he is forced to abandon his life again and again to avoid suspicion. The plot is told as a melancholy reflection on centuries of regret. A deeply human story about loneliness and the burden of eternity. Immortality, it turns out, is no gift at all.