if you were married in 2020, your dad can't claim you if you file a joint return. in fact, it's on the 2020 return that the original stimulus can be claimed. the only reason you might not get it is that the income on the joint return is too high.
in 2019 you say you were under 19 and lived with your girlfriend for more than 1/2 the year. this means that your dad could not claim you as a qualifying child (dependent) because you and he did not live in the same principal residence for more than 6 months in 2019.
he also could not claim you as a qualifying relative (dependent) in 2019 if your gross income was more than $4200
thus based on what you provided and I'm assuming your gross income was over $4200 (you didn't specify who claimed the child), your dad should not have claimed you as a dependent in 2019 (nor your child).
if I am right
what I suggest you do is file an amended return for 2019 with head of household as the filing status and claim your child as the qualifying dependent. do not mark that you could be claimed as a dependent. eventually, the IRS will send both you and your dad correspondence so the conflicting claims can be sorted out. I expect you will prevail and your dad will get a bill.
