Users don’t have permission to delete their forum posts. (We once had a very active member, who in a fit of spite, tried to delete all his comments, making the forum threads incomprehensible. So I took away that permission.)
You can, however, edit the post and replace the content.
So if you make a comment that you regret (as happens from time to time) and want to delete it, simply replace the text with something like “comment deleted.” (You can’t have an entirely blank post.) When I see it, I’ll delete it with my godlike administrator powers. And in a few hours you will be in an Orwellian paradise where history has been changed and no trace left.