It is possible to send a delete request to relays, but you have to give your private key and the delete is a request, not guaranteed. You also have to do it for every post. It isn't worth the time or risk of handing over your key, in my experience (it doesn't really work anyway).

Consider just accepting it and understanding that everything here is permanent. It's a permanent record and that has benefits. I've gotten A LOT better at thinking before posting, which is probably a benefit for society.

No one is going to judge your test posts. You're helping Nostr grow. Welcome.