Was your name legally changed? Or did you use that other child’s birth certificate, and live that ID? : Yes i did use my dead`s step brother`s certificate and lived that ID.
Do you have your original birth records? Just the hospital`s birth certificate.
I don’t know why your GF would drop you because of it - unless you lied to her about other things and this was the final straw. : I broke up with her bcz of many issues just before i go she said tell me the truth about ur name bcz i dont buy it (she believed it all along but some people that r against me told her its impossible and she bought it from them)(also my EX was not mentally healthy (the biggest reason i broke up with her) and i asked her to see a psychotherapist if u want to continue and she agreed to go; but said i will go whenever i want, maybe late in the future, and i didnt buy it, bcz i knew her well enough)