I have found a similar problem with my riding. For a long time I thought
my horse was out of balance on one of his diagonals because I would
twist as I would move forward and up at the posting trot. I had my
instructor ride him and she said they were both very equal.
The problem was me!:(
My abdominals were weak and especially one side of my obliques. So as I
started strengthening the abdominals...both obliques, uppers and lowers,
and doing hip bridges for up to 2 mins at a whack to balance my back
muscles with the abdominals...we're fine now.
It may be your abdominals and your back.
Susan, the gimp and the baby