The depression you're feeling is just a feedback loop because of the withdrawals of quitting coffee. If you stick to not having it for a while, you won't feel depressed like the way you've been feeling. Any addiction takes a minimum of 90 days to fully be sure that your body is back to a normal state. But, if you give in to the cycle before the 90 days, you will continue to have recurring withdrawals probably. Also, it's hard to know whether the depression is caffeine related or coffee related. You need to add up the exact amount of caffeine you're consuming each day to know for sure. For all you know, the coffee you were having was equal to 600mg of caffeine, and to stop consuming that much out of nowhere will really mess you up for a bit.