this is not your horse, right? it seems that you are struggling to find your 'place' in that saddle. I think that it probably has a balance that is totally foreign to you. It looks like it might have a downward slope feel to it, and this is making your balance a bit tentative; struggling against a feeling of falling foward. Thus, your lower leg comes back, and your upper body has to be sort of 'held' back, which makes you stiff and then your hands are not independent.
I have found that some saddles are so different in balance, (and this can be due to stirrup bar placement, or the fit of the saddle making it oriented uphill or downhill) that my riding skill drops down several notches , just like that.
I had a lovely saddle that was SO comfy at the walk, was beautifully made. But, darn it all, I could not for the life of me post well in it. It turned our my friend, who is also fat like me, had the same issue. I figured out that the wide twist, which made it comfy to just sit in, pushed my knees outward, and made it hard for me to get up and over my knees for posting. I sold the saddle to a thinner person, who loved it.
so, the twist can also be a factor in how easily a person can adapt to any saddle.