I'm with MyBoyPuck (and Tinyliny) on this one...not that we need pics, yeah, that would be helpful, but maybe this is a saddle problem. My new lesson horse is 100% horrible in terms of saddle fit. He's a big WB with very high, very narrow withers...and there is only one saddle in the entire barn's inventory that will sort of fit him...and it doesn't fit *me* worth a darn. If I make myself comfy in that saddle, my legs are totally in the wrong position. Not that I'm off balance, but my legs are NOT in the right position. I can't remember which direction they're off (too far back/too far forward) but I do know that if I'm comfy, it means that I can NOT lay a leg on my guy to cue him for a bloody thing. I can't lay a leg on to trot, I can't lay a leg on to bend him, nothing.
I also know that in order to fix this problem, I have to close my hip angle up WAY more than I ought to, and basically lean forward over the pommel (this is supposed to be hunt seat, but forget that, I have to lean way the heck forward even for a hunt position...dressage seat would be totally out of the question).
So yeah, if it's a problem for the saddle fit (on you, not on the horse) then you might need to carry your weight in a strange place. (The issue I have with my lesson horse is enough of a hassle that I've actually considered offering to go in with the owner on a professionally-fitted saddle that will fit me too...even though I do not want to buy this horse.)