How do I teach my daughter to be truthful when I occasionally tell white lies myself?
Good point. I don't think you can. Unfortunately, we can preach all the sermons about truthfulness we want, but our actions always will speak louder than our lectures.
Therefore, the turnaround in your child begins with a turnaround in you. Because most behaviors in kids are caught from parents rather than taught from parents, model today what you want your child to become tomorrow.
It's time to come clean and be honest with her about your own challenges. Let her know that there are times you find it easier to shade the truth rather than tell the truth, but that you're disappointed in yourself. Let her know that as hard as it may be for you, from this day forward you're committed to telling the truth, the whole truth and nothing but the truth.
In addition, maybe you and your child can come up with a code word or a pinky-swear agreement between the two of you mandating that you absolutely, unconditionally, undeniably have to tell the truth. Let your child challenge you just as you challenge your child. It can be something that's fun, as well as functional.