Can I offer some advice?
It looks as though your pinching with your knee over the fences, which is a common thing to do when your whole leg in itself isn't strong enough.
That is also why your back isn't flat and is slightly roached. It is a result of the pinched knee.
The only thing I can say to help this is LOTS of no stirrup work.
Go whole rides without stirrups, LOTS of posting trot with no stirrups.
Also, do a lot of ground pole work as well. Really work on your two-point with no stirrups.
Your ideal leg position without stirrups is a completely relaxed leg, that is wrapped around the horse. Your knee should be on the saddle, but not pinched or tightly. Just resting. Your whole leg should be wrapped around the barrel, with your heel down just as it is in the stirrup.
I can tell your knee is pinched in these photos by the way that your lower leg is flaring out, toe isn't down, and foot is also pointed out lightly.
I think once you get your legs stronger, your back will definitely stop the roaching. You look really good other than that and your horse is very cute!!