Since you asked the question I'll answer it, even if it's not an answer you want to hear. As was mentioned earlier by
@Joshky there are probably less than half a dozen people in this hobby who are vocally willing to help people when it comes to anemone care, who actually REALLY know what they are talking about. You took the avenue of deciding that just because you had some minor success with something you tried, that you would essentially blow off and not listen to a multitude of those voices. You were told this animal needed to be treated WEEKS ago. Instead of listening, you thought you knew better and took multiple other actions.
I know myself personally I knew this was going to turn out like this when you put out your "experienced help only" post a few weeks back. Most of the knowledgeable folks dropped out trying to help you at that point, myself included, because we all saw where this was headed and realized you were only going to do what YOU wanted to do.
@Taylor t was nice enough to try to help you after that, but because of pride you went into minor updates mode and continued to try to battle this based on whatever information you could find which supported what you wanted it to.
So to directly answer your question as to "what went wrong here". This animal needed to be treated a long time ago. You let your pride get in the way and decided you knew better than all the folks who have had dozens of these things and decided to go your own way. So in my "harsh" opinion, hubris did this anemone in and nothing else.
Hopefully you learn form this and I am actually very sorry for your loss.