L.B.
Oh M., I feel for you dear. I have the experience of being on the flip side of your dilemma.
I was a thumb sucker until I was 10 y/o.
Up until I quit, my mom tried everything possible to get me to stop. Remembering the talks we would have about it, I completely understood and agreed with everything she said. Of course, when it was time for sleep, the comfort of that thumb impaired any reasoning we discussed. Not that I was determined to go against my mom's wishes, it's just that this is the way I learned to fall asleep. Period. It's difficult to re-learn something so fundamental... especially the older you get.
The only thing that got me to stop was when I had to get braces. (yes, the thumb sucking was a reason I had to get braces, but also my top jaw grew a lot faster than my bottom jaw... not normal for a 10 y/o to get braces) They affixed a device to the upper teeth that had prongs pointed straight down from the roof of my mouth... looked like a medieval torture device. :( I had to re-learn a lot of things with having this. How to swallow... apparently, I was doing it wrong; how to eat; how to fall asleep because up until that point, thumb was my tool, my comfort, my crutch.
I REALLY wish I had some magical advice that would help you.
The point of this response is more for moral support. You're doing it right. She will remember the talks, the concern, the care and love you are expressing now. Even though the behavior continues, don't for a second believe that your efforts are in vain.
Keep praising her, telling her she's a big girl, etc. It will go a long way, though it might not have the intended effect of quitting the thumb.
Maybe if you take her to a dentist, they might have suggestions for you and your daughter....
I wish you both the best!