The Championship side equalised late in extra-time to take the match to penalties but a miss from Anthony Gerrard, cousin of Liverpool skipper Steven Gerrard, gave Kenny Dalglish's men their first trophy since 2006.
Downing, who picked up man of the match award for his performance, believes the Championship side were unlucky to leave Wembley without the trophy.
The 27-year-old told BBC Sport: “[Cardiff were] very good. Credit to them they’ve been brilliant all the way through the rounds.
“We knew it was going to be tough today. People expected us to turn up and win three or four nil but that wasn’t the case.
“Right to the end they were there and they were a bit unlucky.”
Although in ascendancy for much of the match, Liverpool conceded the first goal when Joe Mason scored early in first half before Martin Skrtel equalised midway through the second.
“We made hard work of it,” Downing admitted.
“Like I say it was a tough game, we ran on empty a little bit at the end but credit to us it’s a very good win at the end.
Downing also revealed he thought Dirk Kuyt’s extra-time strike with less than 15 minutes remaining had won the game for the Liverpool, before Ben Turner netted with less than three minutes remaining, which took the tie to penalties.
He said: “I thought we’d won it when Dirk scored, they were a bit tired. But credit to them they never gave up all day, they kept going.
“It’s a cruel way to lose on penalties but I thought we were the better team.
The penalty shoot-out was just as intense as the previous 120 minutes, with Gerrard and Charlie Adam missing the first two spot-kicks for the Liverpool.
When the Liverpool duo missed, Downing said he thought: “Maybe it’s not going to be our day.
“You’re thinking, ‘If those two are missing what’s going to happen?’
“But credit to the lads, we kept going – we’ve got a great goalkeeper in Pepe [Reina] who’s a good presence and in the end we got the job done."