Hello Friend, Good that you are seeking a help here. Negativity and feelings low are sign of anxiety. Overthinking about one particular incident or future or about career or relationship etc can cause you such feelings and disturbance. Don't worry. It's just like any other physical illness and can be treated through counseling session and few lifestyle changes or dietary changes. Take a professional help to assess your suitation and get suitable recommendations to overcome this problem. Don't hesitate to consult.

There could be some reason behind your disinterested attitude. It is better to seek a professional support. Consult a psychologist in your city personally.