I used to play a dreadnought-size guitar and could tolerate standing and playing with a strap for a while. Then I happened to try a friend's 12 fret 000-size (triple-oh, with 12 frets clear of the body), and I was amazed at how much more comfortable it was to play standing up, with a strap, especially for my right arm/shoulder. Enough for me to save up and buy one for myself. The size and depth of a dreadnought lower bout is awkward for me when playing with a strap. I still have my dreadnought, but my 000 is my main axe now.

So, not knowing what size guitar you have, I'm suggesting that you might try other, smaller guitars to see if that addresses the comfort/shoulder pain issue. For me, it wasn't strap length...

-Greg