Is your guitar a Dreadnought acoustic?

Unless a 6' ecto, the classic Dreadnought's large full depth body is a PITA when played in the conventional seated position for anyone endo or meso of average proportionate build. Worse if shorter than average or have disproportionately shorter torso or arms.

Try using a guitar strap even when sitting.

That doesn't solve the bulky body discomfort problem, but as a workable kludge it will restrain the neck from dropping or moving about as much. 

The better solution is to buy a guitar which fits. e.g. Concert or Parlour body. Not a Dreadnought fan. Do like their sound, detest their fit.