These both seem like informal questions. It would usually be "Are you sure" instead of "you are sure". However, I have heard people say "You are sure we are not forgetting anything?". I think it would be considered slang or improper though. I do not think you should ever use "don't" in this sentence, because "don't" is a contraction of "do not", which is for future tense. The question (I think) you are asking would be either present or past tense. Hope that is clear and helps!