The Toffees' unbeaten start to the season was crushed by a rampant Saints, who scored twice in the first-half to claim a 2-0 victory.
"This defeat doesn't have to affect our belief and ambition," Ancelotti told the club's website.
"We are still in a good position in the table and that is really important.
"I knew sooner or later the defeat would come and it arrived today.
"Now we have to behave properly, keeping the same spirit and attitude and belief and ambition.
"I'm disappointed we lost the game but I cannot forget what we did until now and we did really well, honestly.
"And we have to keep going to continue to do well. This is the point… we don't have to be too disappointed.
"We are still at the top of the table, that is an unbelievable achievement for us and we have to try to stay there.
"It was not a good performance but it came after a really good run for us.
"We found a better team than us [on the day] and they won."