Filing taxes while Divorcing- What are my options? Do I have to file Jointly? We were Denied an Extension.

A nasty divorce and all the headaches to boot. Tax season rolled around in the middle of court dates for protection etc. One of our fights has been how to file. Do I have to file Joint or can I file Seperate? The women in the office where he worked LY screwed up his status and he owes in excess of 5K. He would not listen to me when I told him to get it fixed and I do not think it is fair for me to be responsible for his or their wrong doings. I filled out the paperwork to file for an extension and mailed it out before deadline. Somehow the extension was mailed to me instead of the IRS. I took the envelopes unopened and mailed them together in another envelope. The IRS has denied our extensions bc of the second mailing being after the deadline. What should I do now?