If you are legally married, you must generally file as married (married filing separately) even if you have been separated for a long time. Or if you provide care in your home for a qualifying dependent, you might qualify for head of household status.
There are a very limited number of situation where you might be "considered unmarried" and allowed to file as single, but this depends on the status of your separation under state law. You would need to consult a tax attorney in your state if you think you might qualify. The requirements for this status are fairly strict and difficult to meet. At a minimum, you need a final order of separation from the family court that includes a final order of support or separate maintenance.
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