I have this problem – not just in my thumbs either! All my fingers are double jointed.
Basically, ‘double jointed’ means that your ligaments are too stretchy,
allowing the joint to move out of what should be a normal range of
motion. I have to be careful not to injure myself, so I spent a long
time concentrating on precise technique and not allowing my fingers to
bend the wrong way.
Luckily, as I’ve got older, my ligaments have tightened up a bit, which
helps. Also, as I’ve built up the muscles in my hands, they work to
hold my joints in better positions.
Go carefully, make sure your hands are staying in a strong position and be patient 