verb
BAD : Fortunately, my workload has been lessened.
GOOD : Fortunately, my workload has been reduced.
BAD : The best solution is to lessen the amount of traffic entering the town.
GOOD : The best solution is to reduce the amount of traffic entering the town.
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Lessen is mainly used in connection with pain and feelings: 'I'll give you an injection to lessen the pain.' 'No amount of comforting could lessen Charles' disappointment.'
When you mean 'make something smaller in amount, number, price or size', use reduce : 'The workforce has been reduced by 50 percent.'