If you are still legally married your filing choices are to file a joint return or married filing separately. Filing as "Single" is not an option---and if you have been doing that, your tax returns have been prepared incorrectly. There are a a few states in which a legal separation would allow you to file as "single." Do you have any sort of legal documents to show legal separation?
Or----are there children that are living with you? If so, you could be filing as Head of Household. Please provide some details without any personal information.
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**