I am sure everyone goes through this at some point. Some are able to snap out of it more quickly than others. I find just leaving my work space for several hours does me good - a nice drive does the trick usually. I think about all the negative stuff until I stop thinking about it or I make decisions to resolve the negative thoughts. Many times I just get distracted (kitties and puppies distract me most).
Seriously. Set some time alone so you can work through that stuff, get away if you can, even if it`s for a few hours, go for a drive or see a movie or read a good book (perhaps one of the self-improvement books mentioned here).
You will get past this. Good luck!
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