How Do I Find My Mother?

Hello, now I will tell you my story. There is a mystery around me. For a long time I doubted that my mother was not the real one. After a while, my grandmother told me the truth. My father had a child with another woman - me (she was pregnant then). But my mother wanted to separate them (and she was also pregnant), but my real mother gave birth to me, and her baby was dead and she exchanged them. He is constantly trying to set me against my grandmother, and my father and I seem to have no love for each other. She still doesn't know I know about it. But my question is how do I know who my mother is? Granny says she doesn't know or just doesn't want to say it. I'm just very confused and I don't know what to do ...