You are not eligible for the stimulus payment if you checked the box on your tax return that you can be claimed by someone else. You must check the box if you meet the requirements to be claimed by someone else even if they don't claim you.
If you do not meet the requirements to be claimed by someone else, you can prepare, print, and mail an amended return form 1040X to "uncheck" the box. DO NOT uncheck the box until you start the amended return. Here is a link to help you: https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed
Amended returns must be printed and mailed. They can't be efiled. You must print and mail it yourself. In normal times, it takes the IRS about 16 weeks to process an amended return and is taking longer now because the IRS is currently not processing any mailed returns.
An alternative for you is to wait until you file you 2020 tax return and not check the box on that return. The stimulus payment is actually an advance on a credit claimed on your 2020 tax return. You could get it when you file your 2020 return.
If you CAN be claimed as a dependent ( even if you were NOT claimed) then you are NOT eligible for the stimulus check. https://www.irs.gov/coronavirus/economic-impact-payment-information-center
A2. Although some filers, such as high-income filers, will not qualify for an Economic Impact Payment, most will.
Taxpayers likely won't qualify for an Economic Impact Payment if any of the following apply: