Despite a lot of the other comments suggesting "easy" fixes, untreated and long-lasting depression (in general) and burnout (specifically) can have these lasting effects. These do NOT seem to be treatable without psychiatric and medical intervention - the brain has spend too much time in a depressive state, and it can't get out of it by itself. Psycho therapy alone and other usual interventions (sleep, proper nutrition, positive stimulation) simply won't help anymore.
So, advise for everyone: real (in terms of "professionally diagnosed") depression and burnout for too long (over months and years) can break you in ways you can't imagine. Treat it early.