could vs might

[constantinusphilo]

Sita had tried to persuade herself that the man was mistaken, or lying. For if his story was true, she would have to take Ashok back to his people, which by now was a prospect she could no longer face. It could not be true… or it might not be. She would wait, and do nothing until she was sure.

The Far Pavilions by Mary Kaye

What is the difference between 'could' and 'might' here?

 

[5jj]

In this particular context:

could not be true - it was not possible for it to be true
might not be (true) - it was possible that it was not true.