Ok lots to respond to, I'll try not to miss anything.
My saddle fits great
I'm not hitting myself, it happens even when I do nothing but ride at a walk
I'm not extremely experienced so I'm sure I'm not doing EVERYTHING right
It is possible that my stirrups have been to short
My friend who has done A LOT of training saw me ride today and said my posture and arms were great but my my stirrups were a bit short and she tweaked the way I held my legs a bit
We'll see if there are bruises tomorrow lol
I do have a bit of a naughty horse, he was spoiled rotten and never ridden for 3 years before I got him, he's still in progress (so much better than he was) but it's hard to keep him in one gait, he tries to go back and forth. And when he's ready to go home, the spinning in circles, backing up, and rearing attempts start, went through that today, we got him past it though.
So yeah, my guess would be short stirrups, improper leg positioning, naughty horse, or any combination of those.