say I post something I realize afterward is embarrasing and/or inconvenient (say a password, a friend's phone number, ecc.).
So, you want these edited out - and not be available in any revision of the post, if I understand correctly.
First, edit out the problematic bits (password, phone number, any other sensitive bits).
Then flag the post and explain what the issue is - moderators have tools to redact revisions - so passwords and other sensitive information will not longer be in any revision.
If you are talking about deleting a complete post - that's somewhat different (I doubt a whole post will contain such sensitive information - possibly just part of it - and in that case, see above).
You can delete your own question if it has zero answers or one answer without any upvotes, similarly you can just delete any of your answers.
Again, you can flag such a post for deletion - explain why it needs to be deleted. Note that the issue needs to be more than just "I am embarrassed about what I posted" for moderators to take any steps. Point being - the issue you posted may be of help to others - and when posting on the sites - you have licensed them as cc-by-sa (see the footer) to Stack Overflow, meaning - we have the right to keep them around.