After Alexis Sanchez had given the holders a first half lead, Garath McCleary grabbed Reading a deserved leveller nine minutes into the second period to force extra-time.
The match ended in heartbreak for the Championship side though as goalkeeper Adam Federici, who was otherwise excellent, allowed a routine Sanchez effort to squirm between his legs, sending Arsenal into the final with a 2-1 win.
"It was tough," Wenger told BBC Sport. "They defended well. We didn't find our stride and they were very strong in the challenges.
"We struggled to win the 50-50s and they were really on a different planet in motivation to what we have seen on tapes.
"It's the semi-final of a cup and you never go easy into a final.
"We couldn't find the second goal and, for a team like that, it's the occasion of their lives and they will not give up until the end. That's what they did.
"We controlled the first half quite comfortably but in the second half they came more into the game.
"I think we started flat but they decided to go a bit more on the aggressive side and we started to lose balls that we didn't in the first half."
And Wenger is hopeful his side can defend their crown against the winner of Sunday's second semi-final between Liverpool and Aston Villa.
"When you're in the final you can win it," he added. "Let's prepare well and finish well in the Premier League.
"We come back here and we'll do absolutely everything to win it."
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