All the answers are good so far and hopefully they will help you. It is not uncommon to overthink an issue. You fear now that you are going to start over breathing and become so anxious about doing it, that you start doing it. I have had students who have done this, especially when task loaded, ie; working with a buddy tying a line. The most simple technique is to reduce the task load, or simply put, stop. Just stop what you are doing, concentrate on your breathing and buoyancy and relax. Once you are relaxed and under control again you can start your task again. This will make your task easier and train you to work while relaxed and with a stable platform. In time the break periods to relax will become shorter and less frequent. I hope this helps.