Nicholson was part of the side that failed to recover from Ian Henderson's early opener, eventually succumbing to a 3-0 defeat that leaves their League Two play-off aspirations hanging by a thread.
Graham Alexander's team are three points shy of the top seven in 10th having played a game more than many of their rivals.
"We were really confident going into the game after two decent performances against Gillingham and Southend," he said.
"For some strange reason we didn't get going, we lost an early goal and we didn't really look like we were going to get back into the game, which is really disappointing.
"We've conceded goals in the Dagenham game and Gillingham game, but looked really strong and got back into them quickly.
"Saturday was a really bad day at the office and one that we want to put to the back of our minds. We've three games left and we want to finish with three wins."
Former Preston man Nicholson added: "The big disappointment is when you look at results afterwards and we obviously had a great chance to get ourselves up into the play-offs.
"We spoke about how important it was, we wanted to take it as far as we could with the play-off positions and it's a big disappointment for us.
"It (a play-off place) is obviously still possible and we'll just keep going until it's not. If we can finish with three wins hopefully other results go our way, but it's definitely out of our hands now."