I've been reading about thumb aches while playing barre-chords, and apparently my thumb applies too much pressure, which is bad. Anyway, I kind of figured out how to relax my thumb and my hand. But I'm having hard times on chords that don't require barring the E string, especially those with A shape (B, C, C#, D) I try to free my thumb but it's impossible because otherwise I cannot ring all the string properly.
While doing a barre like G I can properly ring all the strings while my thumb is mid-width of the guitar neck facing the headstock, but on the chords I mentioned it seems really hard.