First, I agree. Do not feel guilty. Often if an egg fails to hatch there i something wrong with it to start with. Sometimes heling them hatch is successful but sometimes they were not meant to make it. Consider it a fatal birth defect.
The hatching process is basically the same for any egg. Before they hatch the chick has to dry up the blood vessels in the membrane that surrounds it by absorbing the blood. If it doesn't do that it can bleed to death if a blood vessel is opened during hatch.
It has to absorb the yolk. It can live off of that yolk for a few days after hatch without needing to eat or drink. That's so the early hatchers can wait on the later hatchers. Occasionally a chick will hatch in its own before it has absorbed the yolk. Some of those make it, some don't.
There are other things they need to do before they hatch. They reposition themselves so they can internal pip, puncture the air cell and live off of that air as they learn to breathe in an atmosphere instead of being in a liquid world. They need to position themselves for external pip and eventually for zip. There are other things.
Some eggs do a lot of this before they even internal zip. Some don't. Some do a lot before external zip, these are nice in that they can zip and hatch pretty quickly after external pip. Some wait until after external zip to do a lot of this. Those can go more than a day before they finally zip. Those can be worrying.
We often cannot tell where that egg is in this process. That makes deciding when to help difficult. If we help too soon we can kill it.
It is not an exact science. Debbie's link to assisted hatch is about the best we can do. I don't assist many but when I do I usually lose about half of them. You gave yours the best chance you could. It was the best you could do.