I always thought I was the only one who could not get a block to be 12 1/2 inches. Mine come up short as well. It does not really matter since I am doing all the blocks, but I never could join in on a round robin or stuff like that.
I usually take a nice break even when things are going well. I am 3/5 of the way finished with my blue log cabin. Oh my, it is looking good. But I am not in a hurry (Jim's Christmas present) so why try to get it done in a day? 3 rows done and 2 rows to go.
Play a computer game, read a while or watch television. Then take a deep breath (as someone else said) and go back in there.