In squatting, breath control is extremely important, but not as important as keeping a consistent routine with the bar. I don't care if you slap your knee 3 times and stomp your feet 3 times before you grab the bar, you better do that every single time you are approaching the bar. That repetitiveness will create a level of comfort that will go a long ways especially when you are going for a weight that is in PR territory for you. For example, with myself, one of the hardest things for me was to contain my nervousness. It is fine to be nervous, but when I let the nervousness consume me, that's when doubt would sneak its way in, and second guessing what could happen came right behind it. I have certain things, certain people, and a certain routine that counters that. I have people that know that when we start getting into the 800's and 900's that they need to be in my ear and reminding me who the hell I am. All they are doing is holding a spark over a pool of gasoline.
It's in me and ready to come out, but sometimes I need that reminder. Then I have a routine that keeps everything fluid, comforting, and routine enough that im not thinking about anything else other than walking out, squatting, and putting it back into the rack. The moment you change something up or do something that disrupts your concentration, you are putting your whole lift and your safety in jepoardy. Bottomline is if I had to give advice for squatting, and/or for any of the 3 major lifts, create a routine in your practice that you will do in competition as well. This will put your mental game on doing your job, then on the little things that should be muscle memory by then. Load the bar and take it for a ride.