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 Pompey.
Pressley said: "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.
"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."
The defeat came 72 hours after owners SISU placed a sister company of Coventry into administration, leaving Pressley's side facing the prospect of a 10-point deduction.
They could also be left homeless when a High Court hearing between SISU and their Ricoh Stadium landlords takes place on Tuesday.
But Pressley said: "There have been distractions, but we will not use that as an excuse."