I would suggest not taking the melatonin so quickly. Your body naturally produces melatonin at night. If you take a stimulus in the morning and then another stimulus at night, your body is constantly needing stimulus instead of naturally allowing to body to do what it does best.
If you want help getting to bed naturally, try meditating by closing your eyes and focusing on calming down your body and breath 5-10 minutes before going to sleep. Then, lay down in bed and meditate while laying down for five minutes. Once you are ready to sleep try not to move so much. The more you move the harder it will be to go to sleep. Relax your body and try not to move. You can also try reading a book while in bed before going to sleep.
If you still are having trouble going to sleep after 20 minutes, then go for a walk and try again. Its best not to stay in bed and force yourself to sleep if you cannot sleep within 20 minutes.