I Would Like to Find the Father Who Contributed to My Life..

I was born in 1968 in Vietnam. From what my mother has told me about him, he was apparently a civilian working under contract for the military. I believe it was the Navy, but unsure because it has been a while since my mother has talked about this.  She had been working in Saigon as a secretary when she met him (his first name is Steven but for some rediculous reason, my mother has said that she cannot remember his last name) and eventually became pregnant with me. By the time my mother found out she was pregnant, my father had finshed his contract work and returned to the United States, New York to be exact. My mother wrote him numerous letters letting him know of the situation but did not receive any response.  In 1972 my mother met my stepfather who was a commander stationed in Vietnam.  They married while he was still stationed in Vietnam, and awaited his tour to end.  At this point, my stepfather, stepmother and myself all left for the United States, where we ended up living at Moffet Naval Base in Mountain View, California, until we relocated to Sunnyvale. At this point I have no idea where my biological father is, and if he is even still alive.  I do have a picture of him while he was stationed in Vietnam.  It is fustrating because I feel as though half of me is not complete.  My stepfather that has raised me never adopted me and ended up kicking me out of the house when I was 18years old.  He never calls me to see how I am doing which makes me feel really crappy about how I feel about myself. I would really like to find my biological father, but afraid that if I do he will not accept me. My mother says that she has none of the letters left, does not know his address or his last name.  It made me really angry when she told me this because, I feel as though she is hiding something. It doesn't matter how many times I ask her, she still comes back at me with the same answers.  At his point I am lost as to what to do. I cannot hire a private investigator due to lack of financial funds, so hopefully someone reading this can give me some suggestions or put me on the right path..