Yes,it is a bummer of a thing,sneaks up on you without any provication.unfortunately it is the nature of the illness. You have to fight it, you self talk, try to fatham out what bought it on, ask yourself all the questions. Move yourself to another room take yourself right away from your present location to break the bad thoughts. If it is a sunny day ,go outside in the sun,it works wonders. Try to engage in conversation with someone,don't continue to dwell on your morbid thinking because as long as you continue to dwell in that miserable thinking mood,the longer it will stay around. Try to do something physical like cooking ,gardening or if you have a pet,hold it close to you ,talk to it ,love it. Keep warm,indulge yourself with a treat like a favourite cake and a cuppa or a piece of fruit you like for that special occasion. Watch a comedy dvd,do that job you have been putting off. Plan an outing call a friend and discuss it. I hope these thoughts help you. If you try to change your thoughts it will pass quicker,if you keep going over it ,again and again,it will drag you down Chin up most of us have been there often ,and we are still here to tell the tale.