Martinez was left to rue his team's profligacy in front of goal, with Jason Scotland setting the tone early on after shooting straight at Schmeichel when put through one on one.
"We just allowed ourselves to get frustrated with the performance," Martinez said. "I think we all knew what to expect in this sort of game; they are going to get a lot of players behind the ball and work extremely hard.
"But we created a lot of chances. Probably the first one was a clear example of the game - in these situations you need to score the first goal and that changes the game completely.
"But you could see that we were getting frustrated and, even though we started well in the second half, we couldn't get the first goal.
"When that happens, there is that risk and you know you have a lot to lose. You are at home and the upset is on the cards, and I think we felt that way.
"But today is a day to congratulate Notts County and allow them to enjoy a huge achievement in going through to the fifth round.
"For us, it is to learn a few lessons, knowing that we have been a little bit low in terms of confidence in front of goal."