Hi Kloeten, an appointment with an orthopedic hand surgeon will confirm whether you have bone degradation or just simple inflammation. I am not a doctor, but have personal experience with left thumb pain. I have some 'arthritis' and have managed to control the pain with 'Aleve'. However, after trading my former very nice guitar and acquiring my current instrument, a Rohe, I was made aware that the string spacing at the nut of the former guitar was 46 mm compared to 43 mm on the one I acquired. In addition the neck thickness of my former was 3 mm thinner. I'm mentioning this because you might want to have your instrument measured to make sure your string spacing is within the norms 42-44 mm and the neck thickness isn't too thin. For me the good news is that I've been pain free since acquiring the Rohe and I installed the same string brand and tension on the new guitar. At the suggestion of its previous owner I've since changed from High tension Ramirez to their medium tension strings. Bottom line. Check your neck thickness and string spacing at the nut. And best of luck controlling your thumb pain.
Richard Brune 'Artist' Cedar/Brazilian 1996