why is the imperfect 'etais' here 'were' when elsewhere the imperfect is translated as 'used to be'
It can be translated as both depending on the context. Imperfect sense just means that it happened for a prolonged amount of time -- In English, it is implied that "you were sad" over a prolonged period of time (days, weeks, months, etc)
She caught him wearing tights.
You were so sad when Camille split up with you should be accepted