McSheffrey's seventh-minute spot kick was saved by Pompey goalkeeper Simon Eastwood before strikes from Jed Wallace and Patrick Agyemang sealed the points for the hosts.
"The penalty was a pivotal moment in the game, but Gary was brave enough to step up and take it so there will be no pointing the finger of blame," said Pressley.
"We started the game well in the first half and had some good opportunities to take the lead. I was disappointed with our passing. I want us to be a team that can dominate the game and we did not do that.
"But I have a really good group of players and I'm sure we will bounce back from this set-back.
"The play-offs are not a bridge too far because we have six games left and will keep on fighting."
Pompey caretaker-manager Guy Whittingham admitted his side were galvanised by Eastwood's spot-kick save.
He said: "A clean sheet is great and to go and score two goals is better.
"Simon did well to save the penalty and that was the springboard for us.
"There are enough games to go for us to stay up."
- Sports & Recreation
- Simon Eastwood