Hold on. Maybe you did something wrong (more on that in a minute) but you need to realize that this animal ran a dangerous gauntlet on its way from the ocean to your tank, and along the way was probably exposed to any number of things that might have made it sick, and made it stop eating soon after it arrived. I suspect that many of the facilities that hold animals for sale in the pet trade spread disease like third world brothels. I catch my octopuses myself, so I don't have any direct experience buying them, but I'm never surprised when I hear about one dying "mysteriously" a few days or weeks after arrival. Shipping can stress them beyond the point of no return, not to mention that they can easily be at the end of their natural life span when they arrive. We've speculated that maybe females who have recently brooded eggs, or senile males, are more easily caught, resulting in a loss of apetite and death a few weeks after arrival in a higher percentage of the animals we buy than would be expected. It isn't logical, or fair, to assume that it is your fault that your octopus is dead. "D" is maybe one of the most careful and experienced octopus keepers on Tonmo, and she recently had two octopuses die on her three weeks after arrival. It makes sense to wonder what you MIGHT have done to cause this, and to correct anything that you suspect as the cause, but we both know that you did very careful research and preparation, and I think that a mistake on your part is the least likely cause of death (20% chance? (WAG))
You probably didn't do anything wrong, and I know you did a lot of things right, but even if it were your fault somehow, I think you would deserve to be forgiven. There's an inherant risk in doing anything difficult for the first time. If you avoided taking any chances, you would deserve blame for missing out on life because of fear. If you had jumped into this rashly, without sufficient study, preparation, and care, then you would deserve blame for taking unnecessary risks. But you did your homework, took it slow and listened to the experienced people. In the unlikely event that you did something wrong, it wasn't because you were rash or stupid, and so it shouldn't be held against you. For a guy getting his first octopus, you did everything right. Any mistake you may have made, after all of your careful research, could not have been a mistake you could reasonably expect to have avoided. A smart and careful rookie should not feel ashamed if they make a rookie mistake that only a more experienced person could have avoided.
Don't feel guilty, just try to discover what you might have done wrong (probably nothing), correct any possible mistakes, and try again.