to sadden, discourage, sink, sap vitality, or lessen value

The words that fit these descriptions are:

1. Depress: to make someone feel sad or downhearted.
2. Dishearten: to cause someone to lose determination or confidence.
3. Demoralize: to weaken someone's spirit or morale.
4. Deplete: to reduce or exhaust something gradually.
5. Devalue: to decrease the worth or importance of something.