Is there a way I can go back in to add another form that was sent to me after I had already submitted my taxes and had them accepted by the IRS?

No, you will need to amend, but don't amend until your original return has been fully processed to avoid unnecessary delays.     

How to amend my return?

You will have to amend your tax return to make any changes.

Before starting to amend the tax return, wait for the tax refund to be received or the taxes due to be paid and processed by the IRS.

See this TurboTax support FAQ for amending a current year tax return - https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-return/amend-federal-tax-return-current-year/L7eS6o1qh_US_en_US?uid=lfunevhk