Just a brief perspective on being depressed with all the work you do for no, or little, results when you are trying hard to close the year on an up note. This issue is not unique to any individual and particularly not unique to a salesperson. All salespeople go through it and the winners recover faster than others.
When the doldrums hit, I suggest, since this will happen again, that you do a sales diary. You will see that the situation, when it does reoccur, will be one you can recover from quickly since you can look back and read what got you out of your last "Oh nothing is going right" mood.
Different industries have different tough months, or a salesperson may just have hit a bad period due to no fault of their own. For many sales people they can get depressed in December unless they see the last month of the year for what it is - a chance to touch base with all clients on a social basis, send out cards, send small gifts of appreciation, and get their office organized for 2009. It is also a great time to reflect on what worked this past year and put together a plan for 2009. What mailings will you do? What new approaches will you implement? Often December is too late to make your number for the year - a lesson to learn and to help motivate your efforts to close more in October and November.
Sometimes just a change of pace will get you back on track.
I recall one salesperson who during a rough period of not generating results but putting a lot of effort into his job, changed his routine and started to read a chapter each morning from a motivational book and then exercising. It got him focused on positive things and not feeling sorry for myself that others were not returning my calls, etc.
I wish you the best for your selling success.