What do I do when I have to breathe deeply while meditating?

I do anapanasati meditation twice a day, and I am overall a fairly tense person. The problem is that in meditation when I feel the need to breathe all the way in, I frequently struggle to do so. I don’t get that inexplicable satisfying feeling that you get after a good, deep breath. I presume it is because I am too tense, but I find myself having sometimes to open my mouth in order to allay my discomfort. I hope I am explaining myself clearly—can anyone help?