Once I started training the horses at shows (young, green ones) I stopped freaking out. It's when I was being competitive that I'd get quite anxious, after 100 or more shows of that, then when I was back to being competitive I was 100xbetter, but still normal show nerves come up.
Generally I just swallow them down. My heart feels like it's going to come out my throat.
Just re read your question and you said before a Comp, not at a Comp! Sorry, I only get excitement before a Comp. But if you do have anxiety before a Comp, there must be a reason why you're unsure, if you can pinpoint that reason/s then I feel like it would lessen the effect of the anxiety. Like are you afraid someone will laugh at your horse, or comment on your not perfectly white breeches. Little things can create huge anxiety, if you address what's making you feel that way and deal with it, by either "fixing" / changing it or by simply practicing to accept or ignore it. Good luck, anxiety is a pain!