You can only file as Married Filing Joint if you live in a state that recognizes the relationship as a legal marriage sometimes known as a Common Law Marriage.
If this is the case with your state you can file for 2019 as Married Filing Joint even though you broke up in November.
If the state you live in does not recognize the relationship as a legal marriage then you should amend the prior returns that were filed as Married Filing Joint. For your ex-boyfriend, he should file as Single and since you have children you can file as Head of Household.
Link about Unmarried Couple Filing Jointly
Link about Amending Prior Year's Tax returns using TurboTax
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