Yes, you must file together.
According to CRA rules, once someone has been something other than single, they can't return to single status again. However, you will find additional information here: (other options available).
Also, the following FAQ on how to change your marital status in online editions of Turbo Tax will be of some assistance: https://turbotax.community.intuit.ca/community/slips-documents/help/how-do-i-do-change-my-marital-status-in-online-editions-of-turbotax/00/873617
I hope this was helpful.