With countable things, use fewer, not less. With things that are not countable, such as emotions and things that are measured in bulk or total amount, use less; for example, you’ll notice fewer lines around your eyes if you use this lotion.
We have fewer stressful situations but less stress. (Stressful situations are countable, but stress is an emotion, so it is not countable.) We have fewer dollar bills, but we have less than five dollars. (Dollar bills are countable, but five dollars is a total amount, so it is not countable.) We have fewer minutes to wait for the train than for the bus, but we have less than 15 minutes to wait for each. (Minutes are countable things, but 15 minutes is a single total amount, so it is not countable.)