The only time you will not be able to delete your post is if it is the first post in a thread ... you cannot delete it then.
That requires using the Report button and getting the admins to delete it. The same holds for changing a thread title- if you make a typo or want different wording in your title, use the report button.
You should be able to edit the OP to replace all of the text by the word "deleted" for example. Of course this leaves evidence that you originally made a post there.