I've been there, and am thankfully out of it. When my 3rd DS was born (after DT's) I spent about 6 months on antibiotics, for various forms of throat infection (laryngitis, pharyngitis etc - in fact if it ended in itis I had it!).
Have you a sympathetic doctor - mine was great and prescribed me antibiotics to keep at home which I could then take as soon as I felt something coming on. I had check ups with her monthly, plus blood tests, and went to see an ENT specialist as well. Try and get your GP on side, don't suffer in silence.
Nothing serious was ever discovered, I was just knackered and run down.
I also went to a homeopath which certianly helped in that I got to spend a long time talking to someone about how bloody awful I felt without them looking at their watch! She also prescribed me stuff, and I took vitamins like there was no tomorrow.
Easier said than done, and I know how depressing it can be, but try and find some time for yourself (early bath once a week, with a drink and a book did it for me), or a walk, or out to a friend for a coffee.
It doesn't seem like it now, but it will get better, and hopefully with some better spring weather it will cheer you up - god knows we all need to see a bit of sunshine.