Probably not if you are still calling her your "significant other." 🙂
The details of common law marriage vary from state to state and you may want to check with an attorney. Three factors seem to be common:
1. you mutually agree to be married
2. you act privately as if you are married (this requires living together in some states and may have other details)
3. you hold yourselves out to other people publicly as being married.
(So when you say she is your "SO" and aren't calling her your wife, it seems that you may not have met condition 1 or 3.)
If you meet the three conditions then you are common law married. It is important to understand that a common law marriage, once legal in one state, is legal in all states, and you can't split up without a full legal divorce with judges, lawyers, alimony and child custody issues. It's not something you can turn on and off like a light switch.
People can live together for a very long time in a common law state and never be married if they never meet the conditions, although, the longer you are together, the more chance that one person could make a good argument in court if something happened.
If you want to be married right now, you just have to agree (#1), check your state for any special rules for #2 (but if you are living together you are probably covered), and post to Facebook or twitter that you decided to be married (#3). That would allow you to file as married for 2017.
But you have to be married as of December 31, 2016 to file a married tax return for 2016. Only you know if you meet the conditions.
This is not legal advice, of course.
