The home side grabbed the lead after just ten minutes thanks to a defensive howler from Michael Raynes.
Another mistake at the back, this time from Damien Batt gifted the ball to substitute Ryan Crowther who drove into the box, finishing low past Wayne Brown.
With the U's already two down substitute Adam Chapman produced the best opportunity for the visitors, his set piece tipped over by Fleetwood's Scott Davies before Fleetwood added a third five minutes from time
"I'm not so sure about the score line," Wilder said.
"We've played a massive part in our own downfall and the goals we have conceded have been ridiculous.
"At any level if you make mistakes like that you're going to be punished and we have been and rightly so.
"We've not done enough."