What is it called and what are causes when someone feels life is useless since they are not their old self? what are some ways to cure this feeling?

Depression??: It sounds like f depression. Psychotherapy sometimes with medication is the treatment of choice. Treatment can be very effective. However when you feel that life is " useless " it is especially important that you be evaluated as soon as possible as there can be underlying proneness to suicide in such situations. If there is any concern about this, 911 can be called or go to the hospital.