I’ve been at this for a decade and let me tell you I’ve hatched lots of ducklings and only 3 of the probably about 8 or 9 sick ones ever made it to adulthood. If I had one with a similar issue to yours it wouldn’t last long. I had one simply collapse mid-step and was dead before she hit the ground. Sometimes there isn’t anything at all you can do to save them, you just gotta make them comfortable and give them some extra TLC. Sometimes they’ll pull through, but usually they won’t. Oftentimes their condition seems to progress faster than they can heal and their little bodies are trying so hard to grow that they can’t get better no matter what you do. Don’t blame yourself. Nature can be cruel to the little ones sometimes. You can only do your best and hope.