I heard that my mother already had me when she met my Father. She still denies this when I ask. It came from my father's own sister! Please tell me how I can find out the truth. I live in NY. How can I find my real birth certificate? MY mom gave me 5 copies and they aren't the originals. I am 45 yrs. Old

That may be really next to impossible since you were born back when adoptions and adopted children's birth certificates were sealed by court order. Open adoption only started catching on in the mid 80's. If you are not getting anywhere with your mother then you might try asking your aunt more of what she knows.

Go to the department in your area that issues the documents and get one for yourself. If you do not live close by you can order them by mail.

Get a home DNA test and do a test ?

Go and get an original birth certificate from the state, you can order one, but if you were adopted by this man, then he will show up as the father on it anyway.

And to be honest what the heck does it matter, was he good to you, did he not raise you for his own ? So maybe some man was a sperm donor 46 years ago, that does not change who your father really is.
Have you considered how he feels in all of this?

And have you considered the sister is lying ?

No, He wasn't a good father. He and his sister have died.

Other than going to people in your mothers past I am not sure what you can do.
As Fr_Chuck said they would have put his name on the birth certificate when he adopted you.
Back then she most likely never even included your real dad on the birth certificate. Back then they sometimes tended to send unwed mothers away to an institution to have their baby and the father may have been left in the dark.
It may be only a secret your mother hid in her past that only she and possibly her parents ever knew.
Back then it was considered a shameful thing that your mother for whatever reasons may have not ever gotten over emotionally herself.