ITR-U New Regime vs Old Regime for returns filed after AY

Hello,

I filed my returns for AY 2021-2022, and AY 2022-2023 under the new regime, and for AY 2023-2024, I was not able to file returns even till 31st December 2023 due to health concerns.

I would now like to file an ITR using ITR-U, but Cleartax shows me that "The income tax department does not allow tax filers to e-file under the New Regime after the due date for filing". Is this true, or should I file under the new regime? If I file under the old regime, I won't be able to file under the new regime ever again.

If I am forced to file under the old regime, will I have to file under the old regime forever? I will pay a lot more tax every year if I do this.

I have a business income, and I have never had a salary income in my life.