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.
“Peace is within oneself to be found in the same place as agitation and suffering. It is not found in a forest or on a hilltop, nor is it given by a teacher. Where you experience suffering, you can also find freedom from suffering. Trying to run away from suffering is actually to run toward it.” ― Ajahn Chah