First of all hello to everyone here. It's a pleasure to be a part of this wonderful forum.
I use the Franco-Belgian bow hold where the thumb is always bent; however I'm having a hard time keeping it bent as after some time of playing, the thumb is straight and I think I got used to that, any suggestions?
I had the same problem and didn't even realize it until I was reading about something else not directly related to bow hold. It wasn't just the thumb though, the pinky was bending in the opposite direction of how it should have bent also.
Here is what I did: I rechecked the entire way of how I held the bow, watched the video on this site of 'holding the bow' (more than once), and practiced it a lot, not only with the bow, but also with pencil and tooth brush and the like. It didn't take that long to fix things, maybe a week at the most.
I hope this helps.
Here is the link:
It's easy to play any musical instrument: all you have to do is touch the right key at the right time and the instrument will play itself. Johann S.Bach