I'm asking this baring in mind the 90 trot I may have to sit to eventually if Lily works out

Whilst she is off at boarding school i've been riding Erin (aka old faithful

) and starting to get her fitter for some jumpies as well as get a bit stronger myself. I haven't ridden much over this semester of uni so need to work on myself before any riding on Lily.
Anyway i'm digressing! I did a lot of no stirrup work tonight (which I can tell already I will feel tomorrow) and in trot I was really noticing the difference in how I could sit when Erin stretched down and softened her back compared to when she was hollow. Obviously it is much easier to sit when the horse works correctly but is it possible on a horse with reasonable lift to pick up the correct movement of the seatbones when the horse is not working so well? I certainly found it bloomin impossible but can that improve with work?
Either way don't think I fancy no stirrups on Lily for a looong time

especially having seen her errr athletic airs above the ground that she has mastered in the field and the loosebox

could be interesting!?!
Thank you!
PS I was going to put 'working through the back' instead of 'stiff in the back' in this title but couldn't fit it

stiff in the back sounds like the horse has problems when I really just mean he is not really swinging.