If you sit there and just dwell on whatever it is then it festers. You need to throw yourself into something like work, hanging out with friends or even exercising. You may not have the energy to do so but just give it a try.
What helps is not to dwell on it but make your mind and body focus on something else. This is easier said than done when in a very dark place but it's still the answer. Exercise or intense work is a HUGE help in situations like yours.