If the tension is the main sensation then just observe the tension, and your reaction to the tension. This is an opportunity to learn, this is the reason we observe the breath, not because there is some spiritual value in finding the perfect breath.
"Proper effort is not the effort to make something particular happen. It is the effort to be aware and awake each moment." - Ajahn Chah
"When we see beyond self, we no longer cling to happiness. When we stop clinging, we can begin to be happy." - Ajahn Chah
"Know and watch your heart. It’s pure but emotions come to colour it." — Ajahn Chah