If you were not legally married as of 12/31/2018 then you only file your tax return as Single if you have not remarried. Or if you have a qualified person as a dependent on your tax return you can file as Head of Household if you meet all the requirements.
See this TurboTax support FAQ for Head of Household - https://ttlc.intuit.com/questions/1894553-do-i-qualify-for-head-of-household
