Like I said before, the double mouth guard I find to difficult to breath with so I use a single mouth guard, which is what my boxing trainer had all of us wear. Anyone who has ever fought knows the importance of breathing and when you get tired you start sucking air, through your mouth!
The things is in amateur competition you have three two minute rounds, so you don't have a whole lot of time to fu**ck around with feeling someone out, you are going to go all out and try to recover as much as possible during the one minute rest period. By the third round, there is a **** good chance you will be sucking wind, even if you are in great shape. The main reason people don't take into consideration is the adrenaline dump. Fighting competitely in a ring is not like sparring, you are in front of a large crowd, where you are everyone's focus. You have pretty much already fought three rounds before you step in the ring due to nerves and adrenaline, and also possible tiredness and fatigue from cutting weight if you had to.
Experience is the only thing that helps with this but even the most experienced fighters still get the jitters and a bit of adrenaline dump, but like I said, when you need air, you need air. You will suck wind however you have to. You will see guys spit our their mouthpiece to try and get more air. It just happens, but hard training and experience will improve this.
"The hero and the coward both feel the same thing, but the hero projects his fear onto his opponent while the coward runs. 'Fear'. It's the same thing, but it's what you do with it that matters". -Cus D'Amato