So every time about a week into a triathlon event
I assume you mean training? Not trying to be pedantic but it’s unclear from reading your post if you mean during training you start to feel anxious, or the night before the event you start to feel anxious, or something else. When you feel anxious might say something about why you’re anxious.
For what it’s worth, I think having some trouble sleeping before an event is not unusual. I’ve seen it discussed in various articles and videos. For me some of it is just feeling ramped up in general or worrying about oversleeping or something.