I can remember that as a child (4-9 years) i was able to hear imagined music in my head.

Now that i started playing instruments over the last 2 years (16-18 years) i noticed that all songs that i make actually resemble songs that i have heard before.

I think that i am using my memory when i try to be musically creative and that is not what i want, because when my career starts it would cause legal problems you know. =)

Is there somebody who knows a little about that kind of training or about how music works in the brain?
How can i train to be musically creative again?
Is it because of a lack of concentration or stress, that i can't be as creative as when i was a child?

PS: feel free to make me aware of grammar mistakes =)