I seem to have a habit of pushing against the back of the fretboard with my thumb, basically extending the fretboard away from my palm. Basically just the tip of my thumb is touching the back of the fretboard. I think I do it to get more pressure on the strings (possibly because my fingers are still weak). I assume this is a bad habit because it really make the muscle under your thumb on the palm tired/cramped. Should I focus on keeping my whole thumb flat against the back of the guitar neck? Not sure how I started this, it is sort of an unconscious thing.