Keeping My Leg Back and Underneath Me

So I just started learning English riding, until now I've ridden in a Western saddle but can't say I've had any real training when it comes to riding. My mother and father both had years of experience with horses and taught me to ride. The rest I taught myself both based on what felt comfortable for myself and what my horse was comfortable and relaxed with.

So there's the background information. Now I only take lessons once a week and the week before last was my first, so I know I can't expect to be a pro right from the start. The trouble I have most is keeping my leg back and underneath me. My instructor placed my leg where it should be so I knew how it felt and it just felt so... Unnatural. It wasn't uncomfortable but it made my back feel really stiff and I had trouble balancing.

Now is this just because I'm not used to sitting in a saddle that way? Could it maybe be something with my muscles (or lack thereof) in those areas? Or maybe it has something to do with my back? I have a genetic back issue that seems to be the cause of accelerated spinal degeneration, when I can afford it I plan to go to a chiro and start rehabilitative sessions to restore my spine to normal, right now the vertebrae with the disc between will look like this '=' the space between the lines being where the disc sits comfortably, my vertebrae sit on my discs like this '>' with my vertebrae sitting together at an angle essentially pushing the disc out and causing pain and what my dad affectionately refers to as "the Horton Hump".

So does anyone have an idea why it might be so difficult for me to achieve this position comfortably? My fiance was put on the horse for the first time in maybe 15 years and achieved a proper leg position immediately and comfortably so I'm feeling pretty left behind. Anyone have any advice or suggestions? Will it fix itself with time and practice? Will my back get in the way of my ever being comfortable? If it's maybe weak legs are there any exercises that I can do to strengthen my legs?

This being my first time ever exposed to English riding I know next to nothing about it and have no idea if anything I'm saying is even useful information. The only other thing I can think of that may be of any import (maybe not) is I'm pretty leggy, my being 5'10" (70in) tall and my inseam is 35in which will give you a general idea of the length of my legs. Aside from my height being 5'10" my weight is about 115lbs.

Any help is appreciated, I feel that it's not comfortable now so it may never be and that's surely a damper on my spirits.

Does your instructor know about your back problem? Chances are that that is the main issue you are having. If your vertebra are resting in the position you indicated, it may make it harder to place your leg under yourself while maintaining a secure seat. I know that for the girls I've worked with (I'm not an instructor), working with no stirrups was the fastest way for them to get their leg under them.

Another thought, are you sure your stirrups are the correct length and the saddle is big enough for you? Too short stirrups can make leg position hard to perfect, and a longer leg (femur, I believe?) requires a larger saddle to get the leg in correct position comfortably......

Ask your instructor to get you standing up in your stirrups for periods of time, first at halt and walk, then trot, then canter, if your back will allow it. Having to stand upright in the stirrups is a brilliant exercise to strengthen your leg and get it into the right position - too far forward and you'll tip backwards, too far back and you'll tip forwards!