How do I resolve this extremely difficult situation with Muslim parents?

How do I resolve this extremely difficult situation with Muslim parents?
I am 31 years old and I am muslim. I have been dating my girlfriend for almost 4 years now who is a hindu. She is willing to convert to my religion. I told her not to convert for the sake of marriage but only if you believe in the religion and then we can get married. I am the only child and my parents are not accepting her. They have never met her and still dont care if she is willing to convert. My father is the problem as he does not want any other culture or converts in our family. He says he has seen it before and it never works out in the end. I do not have a good relationship with my father but i do not want to hurt my parents. I just want everyone to be happy and get along. Im scared that if it doesnt work out then they will say i told you so and i will be disowned and left alone like i have been growing up. I’ve tried asking my friends fathers to speak to him since they have gone through the same thing and i have also tried asking local priests to talk to my dad but he is so stubborn he does not allow them to speak and stands by his points. I have been stuck in this position for the last 3 years and can’t seem to move forward. What do i do?