Oh my....I really feel for you...I've been exactly in your place a few months ago!
We went through this with our 15 year old Red Lored Amazon. We got him as a rescue when he was 10 years old. He came to us on a very poor diet of peanuts and seeds. He had his first vet visit with the complete work up of tests, and all came back normal (no suspicion of fatty liver disease). We were able to convert him to a pellet and fruit/veggie diet with in one month. During those five years he remained healthy and had good blood test results at his annual physical each year.
Fast forward five years to this past early December. He got sick and we took him at the first signs we noticed. He'd lost a lot of weight and his poop was bright yellow. The vet suspected either Psittacosis damaging the liver, or plain liver disease. He was hospitalized and blood tests were done. The test showed extremely high liver enzymes, with extreme anemia. Tests came back negative for Psittacosis. he was just suffering from severe liver disease. He was at the vets tube fed with antibiotic/vitamin shots for a full week. He got a bit better, so we took him home and continued treatment with weekly follow up visits. He did improve for a while, but towards the end of the month he started breathing very heavy and the vets said his chest seemed swollen and after tests confirmed fluid was building up. We attempted to clear it up with a diuretic.
As you we loved him so much and wanted to give him a chance. Sadly he got weaker and we just knew it was his time and we had him put down. It was the most difficult decision we ever made!
I know my story isn't something you really want to hear right now (or at least the outcome), but I felt I had to share it, so you know someone else went through exactly what you are going through. It's very hard and sad, but with time it does get better! My thoughts are with you during this trying time in your life.