Know when to back off. If you are so scared that fear is impairing your climbing, you shouldn't be on the route. Build a base of boldness. You must build confidence through many climbs, while gradually increasing the scare factor. There's no room for fear.
Mind control is crucial.
When I shut off that dialogue and bring myself back to the moves in front of me, the fear melts. Of course sometimes the concerns are very real. In these cases, mindset is everything. I focus the most on staying relaxed, having a relaxed mind. Don't defeat yourself before you start.