I am no expert on why someone should lie, but given that your father was an alcoholic, and knowing how hard it must be coping with that as a child, do you think you might be lying as an instinctive way to protect yourself? Maybe that's how it started and it became a habit. If it is motivated by fear, then that could be why it seems so instinctive and you are finding it hard to control.
I'm only guessing and I'm no expert. Good luck with it all, though. The fact that you are thinking about it shows you have reached a point where you know it's not a good idea to continue without finding out why.
Jule
It's heartening that you are examining this chronic pattern and appear motivated to want to change it. This particular Forum addresses childhood and teenage behavior, emotional functioning, development, etc. You might send along your question to the Mental Health Forum.