Saying your husband made a genuine mistake won't help. Showing you both have good jobs and a good education won't help.
I think you should go to the interview and be prepared to argue that you were not common-law. Say you were just trying out living together to see if it would work, that you hid it from your parents, that you fought a lot, that you broke up several times, that your now-husband left the apartment to live elsewhere for several weeks, that you did not intend this to be common-law as evidenced by the fact you did not have a joint account and paid all your expenses separately. Bring any proof of this you have.
When talking to the visa officer, he or she is not going to believe you, and will argue with you. Try to remain calm, do not give in, do not change your story, be consistent.
Probably the visa officer will find that you were common-law, and so cannot be sponsored now. However, I think you might as well go to the interview and try.
I also think you should start looking into how you could immigrate to Canada on your own.