weak ankles or just doing it wrong?

I recently moved from a jumping saddle into a dressage saddle ( since getting a new horse I no longer needed to use the lessong tack, which was a jumping saddle). My trainer told me how to hold my legs, and I can hold them nicely against my horses sides ( although slightly difficult, my muscles aren't built up). I can do that at the walk, although when I get into the trot , I can't keep them the same way, my ankels seem to woble. I don't know if this is just because my ankels are weak, or if I am acually holding my legs wrong. The only way to take the stress off my ankles is if I hold my legs more like I did in a jumper saddle, but I know that I need to keep my toes pointing forward. Any suggestions?

My first guess is your stirrups are to long, lots of people when they transfer into a dressage saddle lengthen the stirrups a lot longer then they should be. I had the exact same problem and it turned out I didnt have enough weight in my stirrups (i.e. They were to long) :)

It sounds as if in your attempt to 'hold' your legs, you're using the wrong muscles, which in turn is creating the ankle issue.

You don't actually 'hold' your leg...more accurately it should fall from the hip and drape along the horse's sides in a passive manner until such time as you need to cue the horse.

This is correct, but you still need muscle tone to do this, which comes from keeping a still leg. Those of us who have ridden dressage long enough have enough muscle tone to drape our leg and keep a still ankle, it is when you don't have the tone that you get wiggly ankles.
Like ridergirl said, you have to gradually lengthen your stirrups to a comfortable position in order to build up muscle to hold your leg still.
If you still feel your ankle is wobbly then make sure you are riding in regular fillis irons, and look at purchasing a pair of stiff dress boots if you are really serious about doing dressage.
Otherwise, just keep riding and your position will improve if you are conscious of it.

Good luck!

The discussion isn't about muscle tone. That goes without saying.

The point being...'forcing' the leg to be a certain way causes the leg to move.

If the OP concentrates more on relaxing the leg, letting it fall from a more open hip (because she's used to a more closed hip) and riding from her core, the leg will stabilize on its own.

Naturally, certain muscles (particularly the sartoris) will have to stretch over time, but that stretching can not occur by 'holding' the leg. It must come through 'relaxing' the leg. This means that the core must do more work and that's where as a rider the OP needs to get 'stronger'.

You do still need to consciously force the leg to do this! Without the muscle tone, we are forcing the leg because it is not trained to be there yet.