Parma were 3-2 up before losing 4-3 to the Sardinians.
“This defeat will remain unforgettable. It seems impossible we were leading, we had the chance to take the lead again in stoppages and inexplicably lost," D'Aversa told DAZN.
“We keep having these black-outs that compromise the entire match. A team playing for its life cannot afford to give away these goals and fail to convert these chances.
“We had five defenders in stoppages, but allowed four crosses, so evidently Cagliari did better than us."
D'Aversa added: “Cagliari's players came on and they were more determined, ours came on and we lost control of the match.
“Maybe we just don't understand what we need to bring home a result. Analysing all the games, even before I arrived, that seems to be the difference.
“When I was here last season, we could go two days and not concede, because we were so determined and gritty that we would lock everything out. You pay for mistakes at this level and we at Parma have to acknowledge we are throwing an entire season away and it's our fault."