I haven't been reconciling my bank account and it's a mess. My accountant did it in 2018 but I only want it to go from 2021 for tax purposes.

Hello, @pkuroda. 

Thank you for raising your concerns here in the Community. I'm here to share details about reconciling books in QuickBooks Online.

Yes, we must begin reconciling from the 2018 transaction to the present. This is to avoid discrepancies in your books.

If your books haven't closed yet, you can manually unclear the transactions. 

Here's how:

1. From the left panel, click Accounting.
2. Select Chart of Accounts.
3. Find the appropriate bank, then click View register.
4. Select the transaction to edit.
5. Under the Check column, click the box repeatedly to change the status to "Blank". ("C" - Cleared, "R" - Reconciled, and "Blank" - Not cleared or reconciled)
6. Click Save.

You can read through this article for your reference: How to Unreconcile a Transaction or Undo a Reconciliation.

Once done, you can now start reconciling the transactions by month.

If you don't have much time, I'd recommend contacting our support team to get assistance with this matter. They also have tools like screen sharing while they're reconciling your book balances.  

If you have any other concerns, feel free to leave comment in this thread. Have a great day.