Hello, ghostly.

The heart center can feel like the heart pounding. It is often reported to be triggered to pound heavily and fast when the pre-OBE vibrations start. If in doubt, abort your attempt and immediately feel your pulse while staying calm.

About the breath - this sounds like you are tensing up. If you focus on your breath, try not to control it, but observe it. Too narrow a focus may introduce tension, tension reduces volume and create unease, unease creates distraction.

Can you describe how you work with your breath in your own words?

Oliver