You can buy a plug-in diffuser called Feliway, which helps nervous cats. Use this in the room where the cat spends most time. It is very important that you put the needs of the cat ahead of the wishes of the humans. The cat will probably never be cured as such, but with time, patience and a lot of encouragement she may become a bit less nervous. Routine helps. If the cat can be sure of what is going to happen, and when, it helps, and may also help if the cat has her own private space where she can retreat and be sure of not being disturbed.