I often have the same problem. It feels natural to me to hold the bow with a straight thumb, but when I do, my thumb starts to ache within a few minutes.
Holding the bow with a bent thumb, however, feels unstable to me; it feels as though the bow wants to retreat into my palm, and I don't feel that I have as much control as I do with the straight thumb.
I think part of my problem is trying to form my hold while holding the bow in the air. As Joe said, the bow hold isn't firm when the bow is on the string. I guess that trying to control it off the string isn't necessary and can lead to problems like mine.
Through practice, I'm slowly getting used to the bent thumb. Would rubber tubing on the stick make this easier?
Thanks,
Pete