I think a good share of us have gone to do an eggtopsy on what we thought was a quitter only to find that it was still alive. I know I did my first hatch. I was removing what I thought were quitters between day 10 and lockdown, and one was very much alive, (well, until I opened it.) After that I decided I would pull the clears at day 10 and any bloodrings, but everything else was staying as long as it wasn't stinking!
At the end of a hatch when I eggtopsy I put a hole in the air cell big enough to get my little finger in to touch the membrane and feel for movement before I proceed to actualy opening them up. I figure if I am wrong and one is alive it will at least have a chance to hatch. I can't stand the thought of killing another by being mistaken.
You can't help but feel bad, no matter how many other people share your experience, but don't be too hard on yourself. It's a learning process.