That is a very long time for a bird to be on a mostly or all seed diet as such a diet can result in liver disease, which the only way to know for sure about at least accurately is a bile acids test. For that reason alone, I really hope that seeds are not all that you have been feeding her. If she is 15 years old then you got her back when an all seed diet was pretty commonplace, but there has been a lot of progress since then. Although a good seed mix is still or should still be a part of your birds diet it should also be eating fresh fruits and vegetables and whole grains. Whole grains are like seed but are much lower in fat and provide better nutrition for our birds than seeds.
Pellets were developed with the idea of replacing seeds in a birds diet, but due to the main ingredients being seeds the earlier pellets had the benefit that the bird could not just pick out and eat its favorite seeds, so it did provide better nutrition, but it still had many of the same drawbacks as an all seed diet, that being fatty liver disease, heart disease, and kidney disease.
Although some vet's offices recommend feeding pellets, they do so to help to establish a certain level of nutrition in the birds they see. today pellets or seeds are only recommended to make up about 30% of any parrots diet, despite the feed manufactures claims of providing complete nutrition for our birds.
Gloop is a home cooked food for our birds that is made up of partly cooked whole grains, mixed vegetables and fully cooked white beans and lentils. My basic recipe contains about 40% whole grains, 40% mixed vegetables and 20% white beans and lentils. I also feed fresh raw fruits, vegetables and a leafy green each morning just before I give them their gloop and then I give them their seed mix along with a tree nut or two depending on the species of parrot that I am feeding. I have 5 different species of parrots, ranging from parrotlets up to an amazon.
I must admit that after 15 years, that changing her diet will most likely be a project all in itself, but it is not an impossible task. All of my birds came to me as adults and I am still working on teaching them to eat a better diet. For me and many of us it is just one of those ongoing projects that one does, no different than giving them fresh clean water or cleaning their cages.mostly I spend time with trying to get them to eat a larger variety of fresh raw fruits and vegetables. Swithing them from an all seed diet takes some time but it is not all that hard to do, it is getting them to eat a larger variety of raw produce that takes the most time and for that reason I do that first thing in the mornings before I give them their ration of gloop for breakfast. I also share pieces of fresh fruits or vegetables when I have them out for interaction with me.
I would suggest that you look at the list in this link
viewtopic.php?f=8&t=12521 The list begins on the second page of the posting and it contains a lot of useful information about what foods are safe to feed your bird as well as foods to avoid, but this list goes far beyond just foods as you will see. We will be very happy to assist you in any way that we can with your birds diet as well as how to switch their diet toward a more healthy one.