If you have been using a dressage saddle for a long time then you are probably just not used to a GP saddle now.

Have you ever tried a jumping saddle? They have bigger knee rolls to help keep the lower leg in place. I'm not sure if this is your problem but I'm sure it might help. Maybe you could borrow a jumping saddle and try it?

Also, check that the borrowed GP saddle fits your horse (and make sure the jumping saddle, if you borrow one, fits too) as this can make a huge difference to how the rider feels in the saddle, and a poorly-fitted saddle will be uncomfortable for your horse too!

I don't know much about dressage saddles, I've only ridden in one a couple of times but as they aren't built for jumping, I imagine that they aren't very good at keeping a good position during jumping so it perhaps wouldn't be good for your position if you jumped in a dressage saddle unless your position is perfect without the aid of a saddle that is specially designed for the job.

Hope this helps.

You're probably used to  the longer cut of the dressage saddle which to be honest is not really ideal for jumping.  You might just need to work on your position, the dressage saddle has been holding you in the correct position for that discipline and has encouraged you to ride with longer legs, for jumping you need to be shorter whilst still maintaining the correct alignment.  You just need practice!