Hi, Ilikepolkadots,
For different reasons I suffer from some of the same problems as yourself, although my first serious injuries were also caused by a bike/car accident. My double vision is caused by a brain lesion and headaches are of the Cluster type. I also suffer severe Neurological Hypersensitivity.
All this impinges on my ability to reach a relaxed, meditative state, but I noticed in your post that you are utilising deep breathing techniques which trigger problems. At the start of some Buddhist meditation classes one is taught simply to concentrate on your breathing while learning to empty your mind. Do not change your breathing pattern, listen to it and slowly allow it to become the background to your empty mind.
It takes practice, peace and quiet and commitment, but has kept me going through most of the darkest hours and has quelled the majority of anxiety attacks.
Good luck with your recovery.