I found a sickly puppy at my apartment complex my sophomore year and took her to the vet immediately and she had parvo, it was in the later stages where my poor puppy had barely any strength to eat or drink. Sadly, there is really not much you can do after a certain point, i did the same as she did and fed my puppy nutrients with a syringe and water but towards the end when she wasn't swallowing i stopped trying to feed her and just held her close to my chest. at one point when i had to leave the room i found her banging her head against a wall intentionally. after that i didn't leave her for anything. I laid down and she laid on my chest and pet her gently. At first her lungs collapsed, followed by a good minute of her trying to breathe and i was crying and screaming if i should give her cpr or blow into her mouth. At this point i looked at her and knew that it wasn't about saving her, it making her feel loved as she passed away. so I held her head close to my heart and waited for her to die. After a few minutes, she voided her bowels and stopped wheezing for air. She is wrapped in one of my t-shirts and buried in the wooded area by my old apartment complex, where a rock marks where she's buried. I may have only had her for a week but i loved her and hope i did right by her. you just can't save every animal and sometimes theres only a limited set of options and letting the dog die naturally is one of them.