In "Finders Keepers" by Stephen King, there is a character named Andy Halliday and he says his lovelife has been in the rearview mirror for over 15 years.
What does “in the rearview mirror” mean in this context?
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A rearview mirror is for looking behind you when you're driving a car. So to say something is "in the rearview mirror" means that it's behind you - in this context, in the past.
I tend not to spend a lot of time looking in the rearview mirror. If you say, 'Oh, I did 'Hill Street Blues' or 'L.A. Law' and everything I do has to measure up to some preconceived notion of that,' it would paralyze you.
In this context, "in the rearview mirror" is used figuratively to mean "in the past".
Here is another example where it is used figuratively:
With the midterm elections safely in the rearview mirror, Obama is on legacy patrol.
So in the text you've quoted, it means that he has not had a successful love life in the last 15 years.
Rearview mirror, as per dictionary.com literally means
A mirror mounted on the side, windshield, or instrument panel of an automobile or other vehicle to provide the driver with a view of the area behind the vehicle
A figurative meaning that a person has moved on and is no longer having a hold on them.