I know how you feel, I come from a long line of thumb suckers, my parents tried for years to get me to stop. My middle daughter sucked her thumb till she was 12. We took her to the dentist as this was affecting her teeth and she had a slight overbite. The dentist refused to give her braces until she stopped sucking her thumb. He made a plate that fitted into the roof of her mouth (can be taken out for eating) that she had to wear and every time she put her thumb in her mouth it would prick her gum, not painful just irritating and she stopped after two months. Of course she has to WANT to stop as there was nothing stopping her from just not putting the brace in her mouth. But it worked for us.
Can so relate… My eldest daughter sucked her thumb from the moment she took her 1st breath until she was 10 years old. After following every bit of advice given she ventually stopped by herself. Once she realised that its not something 9 year old girls do she felt embarrased to suck her thumb in front of her friends she slowly weaned herself by limiting it to only bedtime. After a couple of months it stopped. I must add it had no negative effect on her teeth only her thumb is flat.