There is a mentality, that if you ride in a dressage saddle, your stirrups should automatically be dropped about 6 holes. Please don't follow that mentality if you are not accustomed to riding dressage yet!
I have been riding soley dressage for years, and am still dropping my stirrups by one hole occassionally, as my seat improves.
It is a gradual process - just like you wouldn't expect a western pleasure horse to work in a highly collected frame and piaffe the day you decide you want to do dressage instead of western. You would gradually accustom the horse to dressage work, building up the required muscles until he was strong enough to begin some collection.
Your own muscles will not automatically adjust to riding in long stirrups, just like the horse not automatically adjusting to dressage.
Shorten your stirrups until they feel comfortable, and gradually work on loosening your hips, not gripping with your upper leg and knee, and doing some work with no stirrups to develop your seat and train your muscles to cope with riding with longer stirrups than what you are used to.