from Jane's letters are that she hasn't been in a good mood for sometime.

In the previous letters she complained a bit about not getting letters as regularly from Cassandra as she expected.

Letter 14 starts with a similar strain ..I have made it a rule not to expect them till they come.

She seems to be sort of depressed.
Cassandra has been away too long. She seems to miss 'dear little George'.
This must have made her feel the gloom of wearing the same gowns, which under other circumstances she wouldn't.
We know how wearing something new helps our mood.