The index cards have become something of a trademark for President Reagan, but in his 37th nationally televised news conference Wednesday night, they were his undoing.

Reagan appeared halting, distracted and confused during the half-hour session in the White House East Room. He misunderstood two questions and gave rambling answers to others, and some of his statements had to be "clarified" yesterday by the White House.

According to a senior assistant, Reagan realized on leaving the podium that he had turned in a bad performance. He turned to his aides and vowed, "I'll never do that again."

What Reagan had done, they discovered, was to devise on his own a new system for selecting the correspondents he wanted to recognize. When the news conference got under way, Reagan got lost, struggling to figure out who he was going to recognize next, and forgetting his answers.

By tradition, Reagan opens a news conference by recognizing the wire-service correspondents and then usually calls on network television correspondents seated in the front row, before moving on to others. All the seats in the East Room are assigned in advance, and Reagan often scrutinizes a seating chart before the session begins.

But this time he decided to try something different.

Before each news conference, Reagan rehearses for hours with aides. Those who were present this week said he appeared to have a good grasp of the material, which included an important explanation of his decision to abandon the SALT II strategic arms limitation treaty. "He was up. He was snappy," said one White House official who was there.

At the end of the rehearsals, however, this official said, Reagan began wondering "why I have to call on the same people all the time."

As a result, the president decided to choose others, and wrote down the names of those he selected on index cards. The names were accompanied by the position of the correspondent in the room and code numbers to help Reagan find the right person, the White House official said.

Reporters who watched Reagan at the session noticed that he was glancing nervously at the lectern. Later, acknowledging to aides that the cards had distracted him, Reagan said: "Next time I'm going to concentrate not on who I'm calling on, but what I'm going to say."