Hi since your age is 20yrs, i believe you won't be having organic cause.
It may be due to recent stress events which is happened for the past 6 months.
Evaluate your stress events get support for your problem either from family or from psychologist.
Next Steps
check your sleep pattern, appetite changes, mood swings, feel about yourself in general( kind of guilty or worthlessness)
Health Tips
Accept your problem and work on it
try to sleep for 8 hrs. read boring book before going to sleep.
Expose yourself to sunlight.