I read this sci-fi novel for young adults when I was a kid, and I would love to re-read it but I can't remember the title. I remember the copy I borrowed from the library had some sort of television with a man's face in it on the cover. Trying to recall this is like trying to recall a dream, but I distinctly remember reading it. If anyone can help, it'd be much appreciated!
The plot involves a girl named Sarah who is close with her grandfather. Her grandfather named her Sarah after a girl he once loved by the same name.
Toward the end of the book, she is with her boyfriend and cuts her finger off but doesn't feel anything. She at some point realizes that she is a robot -- her grandfather had once told her that in order for someone to travel through time, he/she must die, have their dead body surgically transformed into a robot, and come back to being as this robot.
As it turns out, she finds out that her boyfriend with whom she is in love is actually her grandfather because she is really a robot (after her grandfather has turned her into one in the future) and has traveled back in time.