Why we are not provided with the option EDIT and DELETE of our discussions?

You've raised a fair point; I think it would be nice to have that kind of option. However, you could easily modify your post after someone has replied to it, which may make the first responses less fitting (and therefore sometimes deserving a negative rating) which would be unfair. Delete is a very good idea, but they would have to make it so that even if you delete your discussions, you don't lose the profits for it. Plus, I'm not sure anyone would like to delete one of their discussions in case they could have gotten more responses and therefore more profit.

I think there is no option to edit our mistake in our topic we made here. I think mylot will workout that feature.