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.
"Right effort is effort with wisdom. Because where there is wisdom, there is interest. The desire to know something is wisdom at work. Being mindful is not difficult. But it’s difficult to be continuously aware. For that you need right effort. But it does not require a great deal of energy. It’s relaxed perseverance in reminding yourself to be aware. When you are aware, wisdom unfolds naturally, and there is still more interest." - Sayadaw U Tejaniya