Holding your breath is an act of will and when you are tense or emotional an act against your common sense. To learn to relax and breath you need to practice both: For example work against a partner when he is tense and uses power on you and you have to evade and remain relaxed. You can work doing push ups through tensed arms or any other drill you do adding tension to it and becoming aware of it as you work and also use your breath in this. Start walking and count the steps and heart beats you can do until you reach the end of the comfort zone and go a little further. Do the same with push ups or any other drill and lenthen your work within all the phases of the breath and remember that the pause between the inhale and the exhale is not the same as the pause between the exhale and the inhale. Another drill to help you relax and control your own time flow and ability to work is to stand and relax as you hold your breath and go on till you feel a drop inside of the tension you are not aware off and this is a great help to learn to take strikes and keep moving. You can create your own drills from the material found in the book “With every breath” found in the main site and you can add this to your sparring as well. For example you hold your breath intentionaly when you are contacted and can only breath again once you are free. This is a way to feel the fear/tension and learn to use it to your benefit.