Measuring the breathing rate makes it possible to draw conclusions about the user's current mental stress level or physical load. This enables novel opportunities to enhance interactions, treating breathing as an additional input modality. It can support applications designed to actively reduce stress, promote mindfulness, and improve overall well-being. However, it is still unclear how breathing rate can be used as an input modality for adaptive interfaces - especially with the goal of achieving greater mental stress reduction. This demo takes a first step in this direction by analyzing how the breathing rate changes when users are exposed to mental stress using an immersive setup. We isolate mental stress from other types of stress by creating a Virtual Reality free-fall tower application that triggers mental stress by creating a feeling of height. We selected the sitting scenario to minimize the user's movement as a source of sensor noise, a known limitation of many current sensors. We aim to use the measured data to create a Cross-Reality system that uses breath-controlled transitions between stressful and relaxed environments to enable faster stress reduction.