I can't sleep earlier

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From childhood i can't sleep as soon as i lay down in my bed or within 5 minutes. It takes more than half an hour or at least half an hour to start sleeping. Even if i feel sleepy, it will take half an hour to me to start sleeping. When i lay down and try to sleep, many things come to my mind. I prayed to god for peace before lay down in bed. But i can't get peace for sleeping. Even though my mind is in peace, i can't sleep as soon as i try. I don't know whether it is mental problem or physical problem. Any suggestions friends???? Help me!! Thank you!!