My favorite tip is "give your mind a job." Visualization is SO powerful. In between warming up and competing, especially beam, we always take a moment and visualize our routines with our keywords instead of sitting there and coming up with the what-ifs. My coach also helped us create keywords for each tumbling pass so we have something to focus on mentally, while our bodies take over.
But honestly, as you move through the levels, competition has just become less and less of a big deal as you find what works for you. And as it becomes less of a big deal, mistakes become less of a confidence wrecker. I can now say if I start on beam and fall multiple times, I will shrug (dance?!) it off at the meet and perform my best after. Then on the car ride home I'll be ticked, but that is besides the point haha. I have just mentally come to a point where you can't break my chill positive mindset, because its supposed to be FUN. I have had my fair share of meets where I've stressed myself sick and I just wanted it to be OVER, which is not how it should be. When your wound that tight mistakes are even more bound to happen.
Plus, nothing like floor to dance off your disappointment, or tumble away your internal anger at yourself.