See now she doesn't even want the saddle because she associates it with something bad - likely pain. QUIT getting in the saddle. QUIT putting on the saddle NOW. put a blanket on her back after/during grooming to keep the training in place without any associated "badness."
Especially since she is young and still green by the sounds of it, you really need to take it back to before saddling stages. You don't want her to end up with a vice against all saddles.
Find out for sure if/where/why she is in pain. Find out for sure if the saddle fits. Then, and ONLY then, begin putting the correct saddle on her, correctly and DO NOT MOUNT.
Also, posting a pic of the saddle won't help us. It could be a $10,000 saddle but if it doesn't fit the horse, it's not worth 2 cents to you. Maybe, just maybe, you might get input from some really knowledgeable people if you post a pic of the pony and a pic of the saddle. Some people are really amazing about saying whether or not there is any chance at all that the saddle fits her. But it's not going to be the same as someone being there with your pony and with your saddle telling you.