How strange, I've had that exact same feeling of not being able to breathe also. However, I was told on another forum that it's actually the feeling of your heart chakra pounding which gives the feeling of not being able to breathe. Whether that's true or not, I don't know? It feels very real though.
I used to get to this stage too, my whole body would be vibrating and I'd feel like my body was spinning and then up and down and then start spinning again. The feeling of not being able to breathe would always halt my attempt and if I'd concentrate on slowing my breathing down it would stop the vibrations and the spinning and it would ruin the attempt all together. I only ever went OOB a couple of times and then moved on to lucid dreaming as it's easier in some ways than astral projection.

I wish you luck!