hanging on and letting go

I forget...is there an expiration date on feeling bad about something? I don't mean the feeling REALLY bad part, where your emotions become this possessing force that stun you and make you incapable of moving in any direction. What I mean are the lingering bad feelings. The ones that make take you by surprise and make you second guess yourself and your other. The ones that alter your perception, however minutely, so that those little things that flew right past you somehow end up becoming the gnat on the windshield. How long? A week, a month, a year, forever?