I've used this same technique for years. Works well. Your mind is like an undisciplined child, and will resist focus. One way I've found to combat that, and force your focus to remain on your breath, as well as the slowing of one's breathing, is to use a piece of a McDonald's straw. By only allowing yourself to breath through that straw your focus will remain on your breath, or lack of breath, lol
But, in all seriousness, I did this with morning meditation and used it to force my body and mind to accept controlled breathing, which lowered my heart rate. I would recommend a month of 10 minute morning meditation, to get used to slow breathing and strengthen your ability of keeping your focus on your breath before restrictive breathing. Your body will fight it, but you can overcome that and slow your intake and exhalation considerably. I have ADD and some anxiety, and always found meditation to help.