"This relates to a situation that occurred in March of this year which still weighs on my mind, where I was completely out of air in a ripping current about 25 metres below the surface. It required me to buddy breathe from a DM who was only coincidentally close by. My buddy was too far away to help in the current.
I dived later on the day of the incident, and have probably had about 20 dives since, usually reluctantly and nervously. But most days I still relive / remember what happened and when I do it fills me with dread. I have also become far more fixated on reading about other accidents and near misses."
These are classic PTSD Symptoms. That they are still occurring 8 months later. If you were my student I'd advise you stay out of the water until they were addressed by a mental health professional.
I wrote an essay a while back that may apply here. It's in my second book and available in the files section of the Facebook Group I run. It's titled
Post Traumatic Stress In Recreational Dive Rescuers. In a sense, this was a rescue from a potentially life-threatening situation. It was one that was carried out by you and that DM. Even when an outcome is successful there can be after effects for everyone involved that are dealt with differently by each person.
It sounds like you are having some of those and processing them with some degree of difficulty.
Adding a pony bottle or taking another class is likely not going to be a solution. If anything, doing those may exacerbate the symptoms because you are adding additional gear and in the case of a class, performance requirements that may elevate the stress level to the point where a very bad situation could present itself.
Deal with the underlying medical issues.
You are not going crazy.
You are not mentally ill.
You have apparently suffered an injury. A mental injury that is no different than a broken bone.
You would not dive with a broken leg until after it's been treated and healed.
Don't dive with an injured mind until it's been treated and healed.
I have run into similar situations personally and with other divers I've helped to find the resources they need.
In some cultures and areas a mental injury is seen as mental illness and a weakness.
Neither is a weakness or defect. Anyone who has that opinion is one to disregard as ignorant and dangerous.
Seek out a trained counselor familiar with trauma.
Not all mental health professionals are trained to deal with this.
EMDR is one effective therapy and professionals trained in its use are usually suited to help with trauma.
Don't dismiss this out of hand and think that more dives, gear, or classes are going to help.
Even a single session to get evaluated will be more beneficial than advice from those who have never experienced such an event.
Everyone is different in how they process such events.
Taking the advice of someone who never sought help could end up killing you.