Unknown father

This is kind of a long story so I apologize. But I don’t know what to do. When I was 3 years old me and my brother (5 at the time) were adopted by my grandparents. I was still allowed to see my mom whenever I wanted. Up until when I was 17 years old my mom had told me that me and my brother had the same father. I never knew my dad, or even had one in my life. I didn’t even have a father figure. Well, on Father’s Day when I was 16 or 17 I was telling my mamaw I wish I had known him and that’s when she told me we don’t have the same dad. She said she doesn’t know who it is because my mom didn’t know. She said they had to tell someone when they adopted us because all parents had to be notified and nobody ever replied. I’m absolutely furious with my mom for lying to me. She changes her story every time I ask her about it. First she said “you and your brothers dad are the same, but by the way your dad was to your brother I didn’t tell him because I didn’t wanna burden them”. I knew that was a lie because they split up right before my brother was born. I know that because he beat her and left her and sent her into early labor with my brother. She’s told me so many different stories I can’t believe anything. I asked my brother if he knows and he told me a mans name that I have never even heard of but he won’t tell me anything else. Something isn’t adding up here. Also, nobody was at the hospital the day I was born so there isn’t a father on my original birth certificate. Even my adoption papers say unknown father. Someone please tell me how to find out who he is. It’s driving me crazy. I’m so depressed because of this.