The only way it would be legal for your mother to claim you as a dependent at your age, is if either you are too disabled to work, or you earned less than $4200 and she paid more than half your living expenses. In either of those two cases you are legally her dependent and you would not qualify for a separate stimulus payment. Instead, you would be added as a dependent which would increase your mothers stimulus payment.
If you can’t be legally claimed as a dependent—because your mother did not pay more than half your living expenses or because you had more than $4200 in gross taxable income—then you need to file a 2019 tax return in your own name that reports your own personal income and credits and deductions. Do not check the box that says “I can be claimed as a dependent.” That should qualify you for a stimulus payment, although, if you have not filed a return yet, your return might not be processed before the payment is supposed to be made, so it may be delayed. We just don’t know how the IRS is going to handle these things.
Incidentally, if your mother claimed you as a dependent and you do not qualify to be claimed as a dependent and you file a return saying that you are not a dependent, the IRS will be sending letters to both of you to investigate the situation because one of you will have filed an incorrect tax return.
