The advance payments will be reconciled on your return next year. So if you received payments you are not entitled to, you are correct that you'll end up paying them back, and that the mother will receive credit at that time. Your best bet is to use the Child Tax Credit Update Portal to opt out of payments as soon as possible.

Note that if you are single and earn under $40,000, you will NOT have to repay an excess. That phases out up to $80,000, at which point you have to repay everything. So depending on your income, it might not matter.