All I can say is that you just "do". I've been doing "this" for so long that I really don't know what else to say... you take each day at a time and do the best that you can.
My first conure was never "healthy", but his bloodwork never indicated what *was* wrong with him. Sure, his bloodwork wasn't the picture of a healthy bird, but at the same time, it couldn't say "this is what's wrong with him". He was partially blind and going blind with cataracts, he had poor grip, he couldn't fly, let alone glide, he rarely ever had healthy poops... they were always soft and mushy, often times with extra water. He also mutilated himself. I got him when I was barely a teen, actually about 18 years ago now! (right before Christmas!). He passed away nearly 10 years ago. (it'll be 10 come next February)
I also had a bourke parakeet. She was estimated to be about 5-7 years old ("breeder" couldn't remember when she was born). At approximately 14 years old, she had surgery to remove a mass from her foot. Turned out to be cancerous... and it grew back. A couple of years later, she ended up having health issues... which was weird because she'd get sick and nesty at the same time. (she would lay eggs and sit on them while being sick) I'd treat her with milk thistle and DMG, she'd recover and stop being nesty. Once had a sinus infection. That was fun! Had to hold her upside down and flush out her sinuses. Then she was having some other issue. One morning, when she was about 19 years old, I woke up to find her on the bottom of the cage with uncontrollable head twitching. I went to work, hoping she'd die at home. Might be a horrible thing to say, but I *DID NOT* want to make the decision to take her in to the vet. Came home for lunch and she was still alive. Called the vet and took her in. Vet agreed with me... we put her to sleep. Even then, she was fighting to stay alive, but considering age and deteriorating health, I couldn't allow her to continue suffering.
My last cat was about 13 years old and started failing in health. There was no "easing into it". She wasn't acting old or anything, she just suddenly didn't act right. X-Rays were taken and the vet thought they just saw cat litter in her intestinal tract... which seemed rather odd. Well, turns out it was cancer. I separated her from her brother (before we knew it was cancer) and kept a close eye on her. Took care of her the best I could, gave her IV fluids at home. The day I found out it was cancer was the day she had just taken a turn for the worse. Called up family and told them it was time to say goodbye while trying to get more time with her.... even got medications to ease her discomfort... but decided I couldn't prolong her suffering just to get one more day with her... took her back to the vet, same day, for the last time.
I have Beau here now. He was supposed to be just a foster, but for some reason his health just declined. He's sick, he's obviously not feeling well, he's underweight, he's weak... we've been in and out of the vets office getting medications and he's just not getting better. I worry that one day I'll check on him and he wont be there anymore. I've done milk thistle in his water... I've done aloe detox in his water.... I'm now trying some other supplement in his water... and none of it seems to be helping. I guess the plus side to all of this is that despite being sick and unwell, he remains bright eyed and alert! And when he uses something else to scratch his own head he makes the most pitiful little squeaks.
I'm sitting here now just a ball of mess thinking back on some of the critters and all that I did and tried to do for them to make sure they remained as healthy as could be, but feeling like there was something more that I could have done. I feel you are an amazing person for having done everything that you have for Chirp, and continue to do! And you have an amazing vet willing to help out, too! I feel like my own vet, the only one in the area that I trust, has just "checked out" in regards to bird care. She'll still see my birds, but it just feels like the level of care isn't the same. The only other vet I've considered going to in my area does not work on my day off which makes regular checkups difficult... so I feel like the only other option I have is to go out of state, which sucks right now because it's winter and the pass to the next closest vet that I'd be willing to trust with my birds can be dangerous in the winter. (re: Donner Pass) I do keep putting off making an appointment, but if I want a second opinion, I need to.