I haven't played and acoustic guitar for a while, but I wonder if it actually suits the classical position.
I have seen some with much larger bodies (or wider in the middle) than standard classical guitars.
On top of that, fretboards tend to be narrower and my impression is that there is much less space than in a classical guitar.
Also, have you checked you are adopting the right position (when I started, I remember I used to lean the guitar towards my body so that I could see the fretboard, and my elbow would also be too far from the guitar.
If you are teaching yourself, try to keep checking your position with a mirror, and also take breaks if you feel pain to avoid injury.