Guerra, 31, told El Norte de Castilla that Cardiff were willing to pay his £5m buyout clause, something the Bluebirds did to sign Sevilla defender Juan Cala last weekend.
"Friday was a long and busy day," said Guerra. "I made a decision and I think it was the right one.
"I have stayed to help the team and so it will be until June.
"There were opportunities to leave because there was a very important offer for the club and for me. I decided to stay and help my team-mates. It is a major challenge.
"It's true that there was an important offer. Cardiff met the buyout clause and it was difficult to say no.
"I hope people do not think I am crazy. Here I am at ease.
"We waited until the last moment, but the day before I decided to stay and in the end it was a little confusing.
"I do not regret. It was a difficult decision."