I'm no expert, but I have a daughter with ADD. I been through what you feel, and I'll tell you what worked for me...
First, you might consider reading "The 7 Habits of Highly Effective People" By Stephen Covey. I don't agree with everything in that book, but there is some EXCELLENT material in their about influencing relationships through changes in your behavior.
If I were to crystalize it into a nutshell: If you want to influence your child into being a better child, focus on being a better parent.
Spend some quality time with them. Show them you love them and enjoy their company. Look for things they do right and talk about them, at least as much as you talk about the things they do wrong. Throw out any expectations you have about who you want them to be, and love them - sincerely love them - for who they really are.
And always remember... you don't control this relationship. You can only influence the direction - through your own behavior.
Frankly, the very fact that you have posted your concern and are looking for solutions speaks volumes about you. Don't be hard on yourself. You sound like a good parent to me.