So sorry to hear that your little one didn't make it. We had a rather large baby mocking bird fall from our tree a month or so ago. He had most of his feathers, and was probably close to flying age. We took him inside for a day and tried to feed him, but after he refused, we decided the best thing would be to try to put him back into the tree. We watched as he jumped his way back up to where ever he was going. We felt really good about it, but the next morning he was dead on the grass. Poor little guy.
When they are so young, it is really hard to keep them alive. I fear that the fall alone causes internal bleeding because even if they live a few days, it seems as if it isn't good enough and they always die. Out of all the years of finding hatchlings, I think the longest any have lived is 4 days (we had 4 fall out of the tree one time and they died one by one). We have been able to turn 3 birds free, two of which was able to fly within a day or two, and one was an adult that had a hurt wing. Mr Birdy lived with us for nearly 2 months, then one day we felt he might be able to fly and he did. It was the best experience, knowing that had we not taken him in, a cat most likely would have found him.
I find that the best thing to do, is to make them comfortable. We take them in, put them in a box with a towell and cover them loosely. Put a heater near the box for warmth, and we feed them every couple of hours (usually watered down bread). Then it is just a waiting game to see how long they live. Please don't feel guilty, like I said, they are so very fragile and I think that the fall alone must do something really badly to them. I wish we had a bird shelter around here! It would sure save on the heartache. Take care.