When i was young until i was 15 i had an overpowering urge to bang my head off my matress or pillow my doc doesn't know why what could be the reason?

Developmental?: Children who have some type of developmental disorder, especially in the autism & asperger's group of disorders are often head bangers from mild to severe & seems to be a way of self calming. Certain behavior disorders in children such as oppositional defiant disorder & neurologic problems show that behavior. Fortunately you have over come the problem successfully. Check it out with your doctor.

Pillows: I would be more worried if you still want to do this.. It was probably a transitional phase perhaps because you're under a lot of stress. But if you wish to have an answer of something of the past, then usually conditions such as anxiety disorders and OCD will be close in causing this. Not all behaviors have a psychopathology. Just be glad you don't have this anymore.

Self stimulation: Many kids headbang as a way to deal with stress/anxiety or just self soothe,often prior to sleep. It is a process where you control the intensity/duration and "feel" as you get past whatever triggering stimulus you have.It would fade when you develop other coping skills & no longer need the escape. I consider it like self cutting, drinking to excess,risky behaviors.I wouldn't loose sleep over it.