Wallace’s second-half wonder goal could not stop Pompey falling to a shocking 4-2 defeat at Bootham Crescent.
Guy Whittingham’s side served up comfortably their worst display of the season against Nigel Worthington’s side.
The Blues were found wanting for commitment against the Minstermen as they slipped to 17th in League Two.
That hurt Wallace who felt his 50-yard solo effort counted for little in the face of defeat.
He said: ‘This is the lowest I’ve been.
‘I’ve lost the game and it will ruin my week.
‘I was happy to score because it got us back into the game.
‘But then we conceded again.
‘We are shooting ourselves in the foot.
‘It doesn’t matter if I’ve scored a hat-trick. We’ve still lost the game.
‘It’s not good enough. It’s bitterly disappointing.
‘It hurts because we’ve let the fans down and I will be thinking about that all week.
‘But we have strong characters and we have to bounce back and show what a strong team we are.’
Wallace believes he and his team-mates need to reflect on their duties to Pompey’s fans after an inept performance.
He said: ‘We have to put it behind us and push on.
‘A lot of people have spent a lot of money to travel and support us.
‘It’s not good enough what we are doing.
‘We are not warranting the support we get.
‘We have to think about whether we are giving the fans enough.
‘We have to start earning that support.’