Yes, the misinformation out there between vets is very disheartening, especially when it comes to diet. We are always learning, especially about diet. Sweet potatoes and pumpkin are absolutely fine. Stay away from refined foods. Natural sugars are ok too but try to really concentrate on deep colors and leafy greens. Berries are fantastic for them. We are finding that apples, pears, even grapes aren't bad because they have fiber which is good for the heart. Fish in moderation is good for them as well, such as salmon, which is the exception to the rule on the animal products. For too long, we fed egg and chicken. Egg was actually pushed hard, especially to breeding birds and we are really finding that it's not good.
Try not to beat yourself up and don't give up. Your bird is so young and medications can help a great deal as well as a good diet. The other thing you have going for you is that your bird is in great condition physically. Good, human grade seeds such as pumpkin, squash, hemp, even sunflower are fine for treats and training. Find something your bird really loves and only use it for training, don't put it in his regular daily food. Give all these things another year and see where you stand. This isn't the end of the road.