To the OP:

It's not unusual to experience depression (or anxiety, irritability, general mood swings) when quitting smoking. It's withdrawal, your body is so used to functioning with nicotine that it temporarily struggles without it. Nicotine in particular is a stimulant, so when you aren't getting the stimulation you are used to, you may feel low for awhile. This happens even in people without mental health disorders.

I encourage you to talk to your doctor because there are medications available to make quitting more tolerable. other people will taper down their cigarettes gradually rather than going cold turkey, so that the amount of nicotine is gradually lessened that way. Word of caution though, for this to be effective you'll want to set goal dates for when you drop down on the cigarrettes. if say you are a pack a day smoker, resolve to drop down to 16 cigs for two weeks. after two weeks, drop to 14, another two weeks to 12, and so on.

Best of luck