could you try leading her for a while? She might feel more confidant with you by her side - at least then she wont be relying on another horse for confidance.
The more confidant she becomes, the further back you can slip, so she is walking slightly ahead of you - increase this more and more - long rein her if you really have to!
Then, you could try doing it half leading, and then half riding....gradually decrease the distance from the yard to where you get on her until you can get on her at the yard, and walk out without her napping or calling for another horse!
In the mean time, you could try keeping riding out in company to a minimum until she has enough confidance to go it alone too!