I dont think chronic depression is directly related to POIS. Now be warned, some leading/famous docs (that intimately believe in the established medical science) and not familiar with POIS, will much like to caste your POIS into the 'depression bucket'. So dont fall in that trap.
To give my own example, I am chronically depressed, but I dont have POIS. POIS does result in certain magnified depressive symptoms, but they are DISTINCT. Anti-depressants may temporarily "mask" your POIS symptoms, however soon you will realize it is not enough and the 2 are separate beasts.
My 2c, if you need to conquer POIS, try to separate POIS from your many other conditions ailments you may (or maynot) have. Also dont view things as black/white, on/off, instead view things on a "scale".