But I didn’t know how to delete it completely, once I’d realised my mistake - sorry!
That’s okay - there isn’t a way to delete completely immediately - a deleted post will stay there, tucked away, for a while - it’s a way to make sure that admin can have a look if arguments happen, I think.
So don’t worry, you’ve done everything fine 
As much as I could see, hence I’ve deleted few of my posts, too, they stay here for 24 hours not only for admin to take a look in but also that you are able to restore them if you change your mind. After 24 hours they’re permanently deleted (at least that happened to my deleted posts).
How is with topics I don’t know (since I never created one and it’s quite pissible I never will.) 
but deleting topics on all forums are mainly admin and moderator’s stuff to be permited to do. Regular members usually are not permited to do that even if topics are their own. I believe on SSiW is no different but I could very well be wrong though.