Positioning the thumb...?

I'm working on my fingerpicking skills and my thumb is starting to hurt! Ow. Any advice? Where do you place your thumb - you press the strings on what part of your thumb?

My thumb goes naturally to a spot that's on the side of it and near the fingernail. I have a blister there at the moment, not fun. I've tried placing my thumb more on the tip, like my other fingers which are "tip down," but my thumb just won't go that way. Photo included for your viewing pleasure. 

 And no, so far I cannot turn my thumb to press it in a more comfortable way -- but I'll bet somebody here will know a solution!