I recently sent home a client horse after 60 days of work - a 6yo Arabian mare. Highly intelligent, very sensitive...and quite spooky. When she first came to me, I could not hard tie her for fear of her pulling back, breaking her rope, and flipping over. If that rope came loose around her hind legs, she was gone. If there was a shadow on the arena floor, I had to sit deep ;) She took a lot of work but finally left with a lot more confidence, both on the ground and under-saddle! Her level of spookiness had declined, she was walking over tarps, I had ropes dangling between her legs, and I was hard-tying her regularly.
Curiosity has to exist before confidence can be built however so in the horse who is extremely introverted and afraid to be curious, you have to encourage her to start to investigate her environment, etc. All this can be done through groundwork such as the Parelli 7 games and Patterns. Sending (not pushing or pulling) the horse to touch objects and to accomplish specific tasks using approach and retreat.