You file your tax return for each year after the end of the year. The tax return that you filed in 2023 was your 2022 tax return, reporting the income that you earned in 2022. The tax return that you are filing now, in 2024, is your 2023 tax return, reporting the income that you earned in 2023. You will file your 2024 tax return next year, in 2025.
If you were not legally married, it was illegal to file as married filing jointly. It's called an "invalid joint election." Fixing that is very complicated and you will need professional help.
You can file as single for 2023 if you were not legally married as of December 31, 2023. You cannot file as married if you are not legally married. You don't have to do anything special to change your filing status. However, in TurboTax do not transfer information from your 2022 joint tax return and try to change it to single. Start a new tax return from scratch without transferring from 2022, or create a new TurboTax Online account and use the new account.
You didn't say exactly who is "our dependent." If it is a child, and you and your partner are the child's parents, whichever one of you paid more than half the cost of keeping up your home for the year can probably file as head of household. That would result in lower tax for the one who files as head of household, so that's the one who should claim the child. One of you files as head of household, and the other has to file as single.
If you are not both the parents of the child, or the dependent is not a child, post more details. The situation might also be different if neither of you paid more than half the cost of keeping up your home, such as if you lived in the home of one of your parents or someone else. The situation would also be different if one of you is legally married to someone else. So if any of these situations applies, post more details.
