Feet keep sliding forward in stirrup in trot advise pls

[Daisy2]

Yes I have a good seat and we hack a lot but my feet seem to slide forward in the stirrup particularly in trot and I have to keep re-positioning, what am I doing wrong? I have rubber treads and good boots. Any advise welcome.

 

[Cuppatea]

without seeing you ride it would be hard to diagnose but maybe shorten your stirrups and keep your heels down??

 

[vickyb]

Could your stirrups be too long?

 

[Daisy2]

I'll shorten and see how it goes, I tend to get stiff legs if I ride to short