So I first thought it was my saddle too narrow. then upon further inspection, my horse has a bigger left shoulder. So how/what do you do to deal with this?

I have a prs saddle pad on the way, hoping that will make a difference. I am a csi fan so i'm not super happy about this change lol!

He is 18 and was a rope and barrel horse his whole life. I use him for trails and ranch, I don't think I'll compete on him but well see.... I think he's pretty much burnt out, they said he woudn't work in the box anymore and they retired him from roping. but maybe he was in pain, idk.... anywho...

should I try a treeless??? 

get a wider saddle and shim?

what do you all do, they tell me this is common but this is the first case I've dealt with that is this extreme.

The horse is leaning left, balancing over his left shoulder, which has consequently over developed his musculature on the left side.  Learn how to straighten him and this problem will greatly improve over time.  Dr. Deb Bennett has a great piece on her webpage about this issue and how to remedy it. 

CSI has foam shims with Velcro that you place between the liner and the pad. You can make them into whatever shape and size you need. They come separate also so you can just shim one side if you want to. I have a horse like this and ride him in a flextree along with CSI shimmed on one side.

What flex tree do you ride? I'm contemplating a different saddle. 

im not sure there is room for a shim with the shiloh saddle I'm using.