Your stirrups may well be too long, but the main cause may be your lack of a strong core and thus your legs and they way they drape the horse and how you use them.
In H/J Land, all the angles of the body are more extreme - hip, knee, ankle. The legs must support your body over fences, therefore your base of support must be solidly stable in order to easily adjust your seat and upper body. This of necessity creates a shorter, stiffer, more angulated leg with the classic heels down-toes slightly out leg position.
In dressage, the goal is to have a strong and supple core (aka a good seat) with a softly draped leg (think wet linguini) whose muscles are long and supple--exact opposite of hunt seat. You must work on opening your hip angles to allow the active and passive use of those inner thigh muscles and inner calf muscles. A visual of a correct dressage leg should show the kneecaps pointed straight ahead versus East and West, and toes should do the same.
Try this exercise when you first mount, and take frequent breaks to do it at halt and walk, even trot! At halt drop stirrups. Take your leg completely off the horse and raise and swing it
towards the rear of the horse. Then allow your leg to drop softly to the horse's side.
Now with your hand, grab your quadriceps muscle-the long muscle running down the back of your thigh- and pull it AWAY
from the saddle-think of grabbing your inner thigh. Allow the soft adjusted leg to fall softly against the saddle.You'll find that this has rotated your kneecaps to face forward-toes also.
Concentrate on keeping this tension-free leg (you'll feel it in your hip area
) where it lays while repeating the exercise with your other leg. As you do this, keep rolling your ankles in circles, to keep your ankle and lower leg muscles tension-free.
If you have the H/J ankle cock, try working your ankles exactly the opposite of what is necessary for H/J. Think bow-legged; stretch the outer ankle muscles and bring toes in versus stretching inner ankle muscles and cocking toes out. Picture touching your horse's side with the inside of your foot, including your Big Toe
Another visual is to imagine 10 pound bags of sand hanging from your ankles-to keep that long leg.
It always starts with your hips-get them looser by doing the exercises. Moving down the leg, roll the thigh to position your knees facing forward. Then roll your ankles to loosen and picture touching your horse's body with your big toe.
This is all contingent on you working to strengthen and stabilize your core, which is "the core" of a good dressage seat. Lunge lessons will help all of this immensely...but that's a whole 'nuther subject
GOOD LUCK! I hope this helps!