glassortwo I really know how you feel. Dark feelings escalate and can really drag you down. I too have gloomy times when I find one dreary thought seems to snowball into a whole litany of past slights and unhappy events and often ending in really dark memories of great sadness.
I have been reading the 'Mindfullness' book and Ruby Wax's latest offering and, although I am in the early stages of the subject I have found it very helpful to read about the power of these dark thoughts and the damage they can do. What I have found helpful is that these books explain (with useful science based background info) that it is just thoughts, and thoughts are not who we are. I am slowly learning to recognise them, acknowledge them but then say to myself...'well its just a thought so I can unthank it and think of something else. Sounds daft but its beginning to work. I've trained myself to get back to sleep when I wake at 3ish by just going 'don't think...don't think'.
As we get older, those of us with a natural 'glass half empty' look on life cope with so much - hormones, empty nest, loss of parents etc etc. We do need, I feel, strategies to put a little bit of joy back and I do hope you can do that. All the best.