Reincarnation + restraint + fateful love
The male protagonist is the gatekeeper of time. In each life, he can only die before the female protagonist turns eighteen. He has been reincarnated countless times with his memories, just to wait for the day when she comes of age and say "I love you."
She didn't remember him at first, but in her last life she finally woke up early: "Don't die, I'll marry you."
