If you can be claimed as a dependent, you are not eligible for a stimulus check. It doesn't matter whether the other person actually claims you or not. If they could have claimed you, you are not eligible for a stimulus check.
If you cannot be claimed as a dependent for 2020 you will be eligible for a credit on your 2020 tax return equal to the amount of the stimulus payment.
You can't just decide to say that you cannot be claimed as a dependent. There is a set of rules about who can be claimed as a dependent. If you satisfy all the tests to be claimed as a dependent, then you can be claimed.
If you were wrong about someone being able to claim you for 2019, it's probably too late to get a stimulus payment now. For the last couple of months the IRS has not been processing amended returns because of the Covid-19 pandemic. When they start processing them again they will have reduced staff and a huge backlog, so it would be a long time before your amended return is processed.
