Esmerelda I don't know fully the circumstances, only what you've posted on here (and nobody else knows either), but please know that "this too will pass".
What seems overwhelming now, and all too easy to reproach yourself for, will at some stage no longer be a crisis, but a phase that happened, with all the dehumanising that goes on as part of the investigation process.
Keep it in perspective even while those around you might turn it into a witch hunt or who might need to follow the tick box exercise of the process.
Perspective: this is not some evil act committed in malice and in a premeditated way. Your action was not evil nor premeditated but somehow the ball was dropped, whatever. You can't change it.
You seem to have a sensible head on your shoulders and know that realistically, whatever has happened, you cannot turn back the clock nor undo whatever "it" was you did/didn't do. After 30 years, no matter what good you have done during those years, now through the cruelty of that thing called "Life" you're potentially being hauled over the coals.
Keep your head high, cooperate with the process and quite honestly take the rap, if that's required. One step at a time.
They do have to give the worst case scenario, to pre-warn you. It's possible a bullet point buried in their 20 page Disciplinary process. One step at a time, don't prejudge or predict, it will drive you round the bend. So sorry you're going through it 