In the final minutes of last month’s loss to UNLV, several North Carolina players said they panicked. In the final seconds at Kentucky on Saturday, they froze. Last season, the Tar Heels made an identity out of forging late-game comebacks and making clutch, last-second plays. But in two losses over eight days, they’ve confounded even themselves. “I feel like we’re tough enough [to win games],’’ sophomore forward Harrison Barnes said after Saturday’s 73-72 loss at Rupp Arena.