Sounds very much like depression, it's very common especially at this time of the year, yet only a small percentage will address the problem. There doesn't need to be a specific reason, life can be depressing and some people may be more prone to it than others.
Do see your GP. as he/she will be able to help and won't think you are making a fuss. I would be reluctant to take anti-depressants (Prozac etc.) at this stage, see if they can refer you to a counsellor (it doesn't mean you are mad!) or a decent accupuncturist (which I found helpful). Either way you will be one step toward getting better having spoken with a health professional.
I have found that the simple things make a big difference; Exercise, fresh air, healthy eating and cutting back on booze, fags etc. I'm not always particularly good on all of the above but notice a big difference when I am being good.
If you can, print out this thread and take it to your GP. If not, tell him everything you said in your original post. And do it tomorrow. You can manage your depression (which is what it sounds like although I am not a health professional) but like everything, ignoring it will not make it go away.
Good luck and keep us posted.
G