It's disheartening, but the others are right, you have to let it go. Lies are part of life with many of our difficult children. You'll drive yourself crazy if you try to figure it all out, and if your difficult child is like mine, he won't admit to the lie even when caught red-handed.
I also find it helpful to keep a sense of humor about the lies, whenever possible. I'd like to share a story with you.
A few years ago, Oldest was working at a local liquor store (oh, the irony). Her stepmother stopped in one day and when the clerk noticed her last name, she asked if she was related to Oldest difficult child. Clerk then asked if Oldest's sister was ok. Seems Oldest had called in sick that morning, saying that Youngest had been hit by a car while crossing a busy street while trying to skip school (ever notice that the more facts in a story, the more likely it is to be a lie?). Anyway. Stepmother, alarmed, called their dad.. who in turn called our house .. and of course Youngest difficult child answered. She was fine, of course, but furious that Oldest used her in such a bizarre lie. Oldest later claimed that she didn't remember the phonecall (eyeroll), she was half asleep and on painpills.
Anyway.. on to the humor part. Oldest was over at our place a day or so later. We were sitting at the table. Youngest said, "Oldest, can I bum a cigarette from you? I really need one, after being hit by the car and all yesterday."
I laughed SO hard. Oldest, of course, was furious.