Why is my cat so timid?

I have two Burmese cats; one girl, one boy. They are both indoor cats that we have had since they were 6 weeks old. When we first got them the girl was very timid and the boy was very playful. The breeder told us she would become less timid once she became familiar with our family. 4 years later and she is still extremely timid. When we have visitors she will hide behind the bookcase. Around us she is a bit better but will jump at the slightest movement. She is desexed but the vet could only find one ovary when she did the operation. She still shows signs of mating like rolling and meowing loudly. Are all girl cats so timid? How could we help fix this?