Hey Alexander,
Oh yeah the thumb thing, something we all wrestle with at the start. When I start playing I had that same problem. I wanted to stick that thumb behind the board, I thought I would never reach them top keys. Well the good folks on this board said, put that thumb on the side. So I did, it took awhile to train that hand. But in time you be surprise how all of a sudden those fingers reached all the buttons. It just a matter of patience and training that hand. It will take time, but you will get there. Pivot your hand slighty when trying to reach the top or bottom buttons, it does not take much. Your going to work muscles you have not work before in your hand, so it may be sore for awhile.
Have fun learning.
Mark from Arkansas