I can sympathize with you. My daughter finally stopped 4 months ago, but i think a lot had to do with peer pressure.
She came home from school and said she wanted to stop sucking her thumb but she couldn't. She said a couple kids were calling her baby at school.
Now i have been trying for 3 years to get her to stop and nothing worked. I tried the Mavala(spelling) stuff a while ago and still had it, so i put some on her thumb and for about 2 weeks i was constantly getting up at all hours to get her milk to get rid of the taste. THen i came up with putting a sock on that hand so she doesn't accidentally put it in her mouth. She has not sucked her thumb since and she no longer needs the sock and i only did the paint for about a month.
THe callous on her thumb is now gone and she is so much happier, as am i. Good luck, and as much as you don't want to hear this, I know i didn't, but i think a lot has to do with the child wanting to quit or at least the willingness.