All My Befores and Afters — Stephen Donald Huff

He is a secret agent assigned to fight a war of time. He serves an alien overlord, which has subjugated humankind through untold generations. They use him to execute subtle manipulations of time and space by sending him on mysterious errands to faraway places and distant times in the past or future. Each journey erodes a bit of his mind, his memory, his personality, and his psyche. As his personality corrodes, he encounters another alien race intent on aiding his struggle for rebellion. Waging a genocidal war through time and space, can he preserve his sanity long enough to free his people?