You should get some legal assistance on this matter if you are still living together and plan to file any way other than married filing jointly or married filing separately. Colorado has common law marriage. If you hold yourself out to be married, you don't need to get a license and have someone actually marry you to be considered legally married in Colorado. If you are common law married and still live together, you can't file as Head of Household or Single.
Since you previously filed married filing jointly, have a child together and handle your finances together, all of these things could be part of the consideration that you hold yourselves out to be married. To end a common law marriage, you would have to actually get a legal divorce.
You could also be in trouble with the IRS if you previously filed married filing jointly when you were never married and didn't hold yourself out to be common law married.
