I'll put in my two cents. I have the exact same problem. I will lie about big things, but in addition I will lie about things where lying serves no purpose. Honestly, I think I lie because when I was little I got away with lying constantly and never had repercussions. From then on it just became a habit for me, and then like a second nature. Seeing a therapist is a good idea. I have seen therapists in the past and just told them straight up that I compulsively lie. They have worked with me to find out why I lie, and to help me prevent it.
Some techniques I have learned outside of therapy are telling the people who you really trust about your issue. I've come out to my family and girlfriend and it actually feels like a weight is off my shoulders. It was really difficult at first, dealing with the anger and severe distrust. Once all of that subsided, everyone understood that I had a problem, and most people tried to work with it. Setting little goals also works for me. I challenge myself to go a certain amount of days without lying, then reward myself for it (Yeah, it sounds simple and probably dumb, but it does work for me
Seeking therapy is probably going to be your best bet. You just have to try as hard as you can to be honest with them, especially telling them about the lying issue. I find that I lie less when they know I am a compulsive liar.
As to why i lie, i have no idea. I think it just comes naturally to me, i remember when i was younger, i was ALWAYS in trouble. Therefore i had to lie to cover myself, and managed to get pretty good at it then realized that lying was awesome. Then i guess it got out of hand.
But then again, it might be my mum's fault. She constantly used to lie to me and others. She's a manipulative bitch. She told me for years that my real father tried to kill my mother and kidnap me, and fled the country to avoid prosecution. Turns out it was all bullcrap. She got my uncle to come beat me up when i was about 14 too, it used to happen a lot. My uncle got a knife against my throat, i knocked it out his hand and it cut my mums arm. She then rang the police and all the family saying that i tried to kill her. Due to my history of violence and petty crimes, they believed my mum and i got sent to a camp.. Okay i'll stop raging now, i get pretty mad when thinking about my mother. This is my real mother btw, not the one i live with.
I don't know if i can just go and tell the truth, i feel i'm in too deep. Everyone will disown me if they find out some fo the things i have lied about. And they will never believe anything i see again.
How do you stop yourself lying for x amount of days though? I don't even think about lying, they just happen. I very rarely think "I'm going to lie about this", they just come out my mouth and afterwards i'm like "oh crap.. Why'd i do that!?".
I'll try though >.>