As you see from this thread, there is a lot of empathy, because a lot of folks have this sort of experience.
Others simply don't have this experience. I suggest those for whom "Mental Disintegration" or something roughly similar is a problem need PRACTICE to overcome these issues.
It's a little ironic, because the longer the event, the less useful "Pre-Race Arousal" is, but the more likely it is to be a factor. Some folks suggest focusing on the outcome is the reason for being over-hyped, so they suggest folks toss their watches and enjoy. But watch or no these are events that have been trained for and focused on for months, and just being there is gonna get some of us over-hyped to the point that we actually have to DO something to counteract, train for, and deal with that Pre-Race Arousal.
I'm not sure those who suggest they just relax are having the same pre-race arousal experience as those saying they can barely function when it "hits".
A mass swim start of 1800-2400 folks isn't a particularly "normal" activity, and to control over-arousal I'm suggesting folks practice warming up, treading water and allowing the heart rate to slow, and then swimming, because practicing this can help us move forward with our own plan on race day.
And in every aspect of the event "moving forward with our own plan" is the important part.
Having a very specific race day plan, one we can train for executing, just like we train for the other aspects of the event, allows counteracting pre-race arousal to become just something else we DO, not something that happens to us. We warm up, we tread and our HR slows, we swim- both in practice and on race day. While practicing relaxing, if we desire, we may develop a check-list of swim technique, sighting lines, positive keywords or mantras to be repeated, ANYthing that we can do in practice, and repeat on race-day to normalize the event.
Swim start is great place to do this, but the idea holds true for any spot in the event, where we can reset, re-checklist, and re-focus on what lies ahead, leaving the last segment behind. That way, if we're walking or even crawling at the end, as long as we're thinking of it as this next segment's "strategy to finish", and not something that "happened to us" we are moving forward with our own plan, and therefore succeeding!
Tally Ho, and headlong into the Fray.