Second-half goals from Bruno Berner, Lloyd Dyer and Matty Fryatt ensured that Crewe went down but Thordarson called for the fans to stay with his side.
He said: "It's hard for the fans because they are bitterly disappointed and I understand because relegation is not my cup of tea either.
"But they need to have faith, stick with us and they need to know I will accept nothing less than promotion next season.
"We knew the game was going to be tough because we were playing the best team in the league and we all saw the difference.
"I've learned a lot about my players and what we need to improve. The preparation for next season has already started and there are lot of things we need to improve. We will need to mix it up in League Two. Some players need to get out of goo-goo land and get in the real world if they are going to succeed there."