Hello, I just realized I forgot to add a temporary second job I had in my taxes. What should I do?

You will need to amend your tax return and include that extra income.  ALL of your 2021 income was supposed to be on your tax return.

https://ttlc.intuit.com/community/tax-return/help/how-do-i-amend-my-2021-return/00/2544441

You will have to amend your tax return. Depending on the amount and type of income, you will either get a refund or you will owe taxes. It is very important to include all your income to avoid any penalties and interest from the IRS.  Please visit How to amend (change or correct) a return you've already filed to see the steps on how to amend your 2021 Tax Return.