What causes unhappiness and hopelessness? just low serotonin? what foods increase serotonin?

Medical vs psycholog: There are both mind and body causes of unhappiness and hopelessness. Low serotonin, hypothyroidism, and other medical conditions can impact mood. Psychologically things like loss, significant change (positive or negative) or other factors can impact mood in a negative way.

Rich complexity: One of the gifts in being human, is that we're rich, complex beings -- far more than neurotransmitter levels alone. That said, what we eat affects us profoundly and can even modulate what our genes do. You want a balanced diet of proteins, veggies & fruits, and healthy fats (like fish, avocados, olive oil). Please talk with a therapist about your life -- see what changes may help your outlook.