Harriers went down 3-0 having held the Ammies at half-time - watch match highlights here.
Joe Ironside had his penalty for the visitors saved in the first-half, and then substitute Jack Redshaw, who came on at half-time, netted a hat-trick after the break.
Kidderminster have won two, lost three after losing in the play-offs last season.
"I was just really disappointed with the second half and the way we came out," Eustace told the club's website. "When we conceded the goal our heads dropped straight away, I need better than that and the team needs better than that.
"We need a few more characters. The way we dropped our heads isn’t us and that’s something I’ve got to sort out. Goals change games and they had a lot of momentum in the second half.
"We’ve got to work on being a lot more focused at the start of the second half.
“We’re five games into the season and we have got a lot to work on. We're got a new back five and lads that haven't had a pre-season. There’s lots to take into consideration but we want to work harder and get better.”