I sympathize with your problem! Part of the problem is that you have been given advice that doesn't take into account that people are not all built the same, and that there is more than one way to get a good result. So many people supposedly "in the know" know one way that works well for them or various, or even many, other people, and assume it's the only way. Amazingly enough, not too long ago in America (less than 100 years ago) people who were left-handed were taught that wasn't acceptable and had to write right-handed. (Stupid!) This unfortunately carries on with various dogmas in banjo society.
My only dogma is: It has to SOUND GOOD.
I assure you there are many ways of striking a string with a thumb pick that sound good. You need to find the one that is comfortable while producing good sound. It takes experimentation and patience. Start with just slow, single hits with the thumb, with your hand in a comfortable position. Move it around various ways, still playing JUST your thumb, on different strings, slowly. As soon as it sounds good, note the position. This might well be the answer. Of course, the other fingers also have to be positioned to achieve good sound consistently and comfortably. Trial and error will give you the best answer. Then stay with that as you slowly play rolls, then some slowish versions of some of your easiest pieces. Don't accept sloppy sound! It might take a while for everything to fall into place, but don't pick an uncomfortable position in hopes that it will get comfortable. May or may not happen. If you find a position that seems to work but is awkward, you might try it for a while to see if it gets easier. Avoid pain!
I think this will work for you before very long if you don't rush through it. Just stay with it, and know there's a good solution waiting.
Once things seem to work cleanly and comfortably at slow speed, gradually move your speed up in pretty slow steps (like 5 beats/minute at a time, never more than 20 on any given day). Keep it clean, keep it comfortable, or don't speed up! Over a period of time, this correct and comfortable position will lock in, and playing faster and staying clean will be an attainable goal. Don't forget, everyone has trouble playing fast and clean, and it takes practice!
If you feel you've truly hit a wall in spite of following the above advice, it's time to go to a hand specialist and see if the problem is actually physiological. But I think that is quite probably not the case.
Please let me know how it works out.