Very glad to hear that you are ready to deal with this. All kids lie, but most grow out of it. Can you 'come out' to a couple of key people? Admit that you know it is a big problem and unfair to them--and make them into allies in the change process.
The "Why" question is important. Why do YOU think you do it?
(Keep in mind that knowing that answer is useful, but only one step toward stopping.)
tg