According to daniel stewart, a celebrity trainer / physical-riding therapist / equestrian psychology teacher etc...
ANYWAYS. He gave some crazy numbers like "3/5 riders have lateral balance problems (what you have). And like 3/4 riders don't always keep a proper vertical line (when viewed from the side). This guys works with top level eventers and schooling kids and everything in between so that's a pretty eye opening
statement. His clinics involve a TON of video analysis (and a psych lecture that'll make you laugh and feel better about riding than ANYTHING) in between the 2 main riding sessions so he gets into that stuff a lot.
You probably drive... your left leg is held in to your left and is always kept a bit stiffer than your left. Your left probably hangs out with knee a bit off to the left (opened) and is TOTALLY relaxed. That alone can make major imbalances.
There are physiatrist (not physician), sport medicine physical therapists, and the new thing is these kinesthesiologists (movement specialists). My chiropractor didnt know what kinesthesiologists are but I keep hearing it like its the new thing. Also my chiropractor only wanted to crack my back and make me lay on stupid roller tables. I ended up with new pains in my back in the places he cracked that are still trying to heal (quit a month ago) the sport med physical therpists most likely won't have a clue how to REALLY pinpoint the imbalance and fix it... trust me. Lol. Unless its a true equestrian therapist, which are for the rich people. I would try a physiatrist first, then a kinesthesiologist, then maybe a sport med PT. Also there is an amazing thing called "The Feldenkrais Method" and "The Alexander Method". There are books, videos, classes for those. They will probably help you more than ANYTHING. And help you never have back, knee, etc problems ever also. Compared to those.. yoga is like childsplay. Not to say they're harder than yoga, they will teach you more about your body than yoga and find these strange imbalances and fix them... easier than yoga! :)
Go here and click about him to see how awesome this guy is... I've seen some of his stuff and its amazing! Daniel Stewart | Ride Right - StewartClinics.com