Sometimes I find in hard to find the right seat. When I sit back I feel like my hands aren't far enough forward and when I move my leg back I feel like I'm leaning too far forward.. does that answer what you're asking?
Have you tried lowering the stirrups 1 hole? I think that would help.
Also I can see that your horse is willing to work with you without any rein contact. What would happen if you were to sit back and take up that slack?
Also if you look at the picture you provided, your hands are too far forward so you're having to lean and compromising your seat, and your leg therefore shoots back.
Be hypothetical with me for a minute. If you were to drop the reins, without worry that your horse would run off or do anything other than stand, and sit back with a nice straight back (not stiff, just straight instead of arched) where do you think your hands would be?
I picture them exactly where they should be, but with your leg out in front of you a bit.
What do you think?
Now if you were to take your feet out of the stirrups and stretch them down, where would they be? Behind the girth? On the girth? Where the stirrups are?
I think it would benefit you to take your stirrups down a hole, and work on your 2-point to get those lower legs underneath you. If you're taking lessons, I would ask to be put on the lungeline to do no-hand riding.
I think you feel off kilter due to not being balanced over your horse, which will change once you start to work in two point.
I also encourage you to watch this video, it helps with the "classical" position but it offers tips to fix common rider position errors like leg too far back or leaning forward.
Do you warm up stirrupless?
Also open up your chest and ride with your shoulders down and back. Use your core for balance.. use those seatbones :)
Hope I helped.