To file as single, you must be “considered unmarried“ under the laws of your state. This is not merely a legal separation. I have only researched New York State law, but in New York State, to be considered unmarried, you must go through a legal separation process that is still contained in New York law, but is obsolete for all practical purposes. It is a holdover from the past when divorce was very difficult to obtain and when people took religious proscriptions against divorce more seriously than they do today. It was a legal process that resulted in a declaration that the two parties could lead separate lives and had no legal or financial ties or powers or responsibilities over each other and were separated in every way except they could not get remarried to anyone else. Since this process costs just as much as a divorce and is just as painful, but does not allow people to remarry, no one actually uses this process any more. The tax court has specifically rejected the argument that an ordinary legal separation in the state of New York allows taxpayers to be “considered unmarried.“
You may want to consult with an attorney in your state. However, if you are still legally married, and can’t be “considered unmarried“ according to the laws of your state, then you and your spouse must both file as “married filing separately.“ If you really want to create problems for your ex, then simply filing your return as “married filing separately“ may trigger an audit of their return. Alternatively, there is a process to report tax fraud.