It's been a year since I quit smoking, but this year has been one of depression and counter-productivity at work. Is it common to experience things like this? If so, how long should it last?
A recent review looked at 26 studies that examined mental state after smoking cessation. The review found LESS depression among people who quit, not more.
Anything is possible, but based on this, it would be best to look elsewhere for the cause(s) of your complaint.
Also remember that depression is a very treatable disorder, and the treatments are pretty safe. You wouldn't take a depression-treating pill if it had a 50% chance of killing you, so it would be even more unwise to start using tobacco in the mistaken idea that it could help your mood (it has a 50% chance of killing you).
Also note: schizophrenics are the ones who self-treat with nicotine, not depressives.