I'm going to jump off the deep end a bit here, but maybe some of my drivel will make sense and help. A long time ago, in a galaxy far away, I had a professional practice as a clinical hypnotherapist. Hypnosis is all about the subconscious mind, so the training goes into how the subconscious mind works in some depth. I also had the opportunity to work under a Jungian-trained psychotherapist for several years, and she explained a lot about how dreams reflect what's going on in the subconscious mind. So ... on to the case at hand:
Camolicous, if I'm understanding you correctly you used to shoot a lot and you never had trigger freeze. Then one day you had it. So the fundamental question is -- where did it come from? Generally a deep-seated phobia like that doesn't just happen -- some event in your life made a strong impression on your subconscious mind, and your subconscious mind now tries to protect you from what (it thinks) will harm you if you pull the trigger.
Can you remember approximately when the fear first materialized? If you can pinpoint that, then take some time to sit down alone and in quiet, and try to remember if anything (ANYTHING!) happened shortly before that that could in any way be interpreted by the subconscious mind as a warning that shooting a gun is dangerous TO YOU. It doesn't necessarily have to be something that happened to you directly. It could have been a newspaper article you read, a news report you saw on the telly, or perhaps something you saw in a movie. It's not totally necessary to identify the causal event, but it would be helpful.
Next, I would like to suggest that you do some dry firing. Not at a range, not in the basement. Again, be alone in a quiet setting. [Edit to add: BE SURE THE GUN ISN'T LOADED! CHECK IT AGAIN.]Sit in a comfortable chair with the gun, close your eyes, and cock the gun. Take a deep breath, and as you let it out you can feel your body relaxing. Tell yourself, "I am safe. I am completely safe." Then pull the trigger. Don't do it "right." No squeeze and "surprise break." Just yank it -- make the hammer fall. Then say to your subconscious mind, "See? Nothing happened. I'm safe when I pull the trigger."
Repeat this -- many times in one session, and then repeat the session every day for a couple or three weeks. If at any point you feel your body beginning to tense up -- stop pulling. Take another breath. Let it out and feel your body relax. Repeat five times, THEN inhale, exhale, say "I am safe. I am completely safe," and just yank the trigger. "See? Nothing happened. I am safe when I pull the trigger."
Yeah, yeah, I know. I'm telling her to practice horrible trigger control. Trigger control can be re-learned, but she can't improve trigger control until she's ready and able to pull the trigger without freezing.
It's possible that in the course of doing this exercise you'll remember what it was that caused the fear in the first place. If so, don't block the memory. Embrace it, and examine it carefully so the subconscious mind will understand why and how that event was different than when you want to shoot. Look for any and every aspect of whatever the event was and count up the ways in which your shooting today does NOT relate to that incident.
End of lecture. make any sense?