Cathcart allowed Elvis Saric to nip in and score after mistakenly heading the ball back to Bailey Peacock-Farrell.
"Obviously we're really disappointed with the goals and I'll take full responsibility for the second one," Cathcart said.
"I maybe didn't put enough on it and we are just so disappointed given the rest of the performance.
"How the goal came about is just one of those things in football. It won't be the first or last mistake I make; it's just part of the game."