t12kim, I can tell you are the type of person who can never accept that things are "good enough". You lay awake at night worrying about trivial little details that other people would never notice or care about. You are rarely if ever satisfied with your own performance. Congratulations, you have all the makings of a great dentist.
But in this case it is time to just relax and be proud of your achievement. 21 AA is awesome and going to get you tons of interviews; most schools won't even take a second look at your PAT. Retaking that monster isn't worth the stress, time, or risk of getting a lower score.