The ratchet is sticking. Get a can of carb cleaner and spray it into the internals of the shifter, then action the shifter a bunch, leave the cable loose and hold it snug with your hand. Do this a couple of times and the ratchet should free up. After you free it up drip some light oil in there to keep it smooth. If it does not free up the ratchet spring is broken and you will need to replace the shifter.
Be careful with the carb cleaner, it is hard on your skin and eyes.
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I am in the woods and I have gone crazy.