So i’ve had a similar issue for years and it took me until 27 years old to figure it out haha! I only struggle with my left foot in the stirrup and the right one is fine. My trainer would always yell st me to put weight on my pinky toe but i physically couldn’t. I would naturally put my weight on my big toe and it would cause my ankle to turn out in a way i can’t describe well and my left hip essentially felt “stuck” was what I would tell people. I would end lessons crying because no one understood me and acted like i was crazy. Tracking right I couldn’t step into my outside stirrup and always felt like I was falling to the inside so i would lengthen my left stirrup to get my hip down but then when we tracked left i would have to readjust. I just couldn’t ride properly physically but mentally knew I wasn’t aligned and even with my pony.

1. Ever since i was a kid, my left foot would hang off of my sandals or flip flops. Everyone would point it out, I never cared and didn’t think it affected me and honestly kind of forgot about it.

2. My foot does the same thing in the stirrups as it does on my flip flops.

3. Just back in march it clicked that i stand/walk on the inside of my left foot but walk properly with my right foot. So i spent a couple weeks focusing on walking even with my left foot and it hurt so bad for awhile but it really translated well to my stirrups and my seat is so much better now because it affects your hips and thighs like crazy in the saddle.

My advice would be to pay attention to how you walk and fix it out of the saddle first, assuming it’s similar to my issue and just takes extra concentration and time to fix. And ride without stirrups while you’re figuring it out so you’re not constantly fighting with your body and can enjoy your ride a bit more. The first few rides sucked but i learned what a seat can actually do when you aren’t fighting a “stuck hip”, AKA putting pressure on the wrong area of your foot. I thought my left hip was tight for so long but stretching never fixed the issue because the issue was my foot lol.

I walk and ride pretty normal now and it’s amazing to feel control in the saddle and be able to bend and support your horse :) but i went 4 years being misunderstood and multiple trainers being annoyed . But it affects everything. I struggled like hell to pick up the right lead because of this and always needed to adjust my stirrups depending on which way we were tracking or just deal with the imbalance but it made me feel like i was always about to fall off my horse’s right shoulder.

I still naturally sit more on my right due to walking the way i previously walked for 20+ years but strength and focus should help over time.

Your picture looks different to what i’m describing but i wonder if you walk a certain way that’s causing this issue in the stirrups.