You need to get that fitter back out. Unless your horse is horrandeously underweight at which point you shouldn't even be riding your horse, there is no reason why your saddle shouldn't fit. Did they tell you what they did to it to correct it?
Some saddles can only be altered and changed so much, so I could see it possible that you might have spent the money to get it corrected more and it wasn't enough to fix the issue with your mare. Having said that, your fitter should have been able to tell you before it was "corrected" that there was a good chance it wouldn't be enough to correct the issue.
I would sell your saddle and get a new one.