Since you already filed a joint return with your partner, she cannot just file a single return. The joint return that you filed was for both of you, so she has already filed a tax return. You and she both have to file amended returns. But amending from a joint return to two separate returns is a difficult and complicated process. You can't just file a normal amended return.
Normally you are not allowed to amend from a joint return to separate returns after the original due date of the return. "Separate returns" means anything other than joint, including single or head of household. If you just file normal amended returns, the IRS will send them back to you without processing them, saying that you cannot amend from joint to single. But your situation is an exception to the general rule, since you were not married. Your joint tax return is what is called an "invalid joint election." You have to amend the incorrect joint return, but a special procedure has to be followed, and there are complicated rules for additional documents and information that have to be submitted with the amended return.
This is a very uncommon situation, and it's a very obscure part of the tax rules. You will need to have a tax professional prepare your amended returns with the attachments that the IRS requires. You probably won't even find a tax pro who is familiar with this particular situation - it doesn't come up very often - but look for someone who has experience dealing with the IRS in unusual cases. You can tell the tax pro to refer to IRM 21.6.1.5.7, which is the section of the Internal Revenue Manual (IRM) that covers an invalid joint election. When you meet with the tax pro, bring a copy of the joint tax return that you filed.
