Bless you, OP, I do understand how you feel. Even though my extended family is very multi-racial (eight mixed race aunts and uncles, for a start) I remember having a little odd wobbly feeling when I was just getting involved with my (black Caribbean) dp, many many years ago. Just a feeling of, "Oh. Is this going to be very different?". And then a similar feeling on adopting our mixed race dd.
The little wobble was very quickly drowned out by the busy reality of falling in love (with dp, with dd) and getting on with life. And discovering that, of course, the core experience of loving someone is irrelevant of racial identity. I don't mean that love is colour-blind - it's not; their racial identity is one of the things I love about them, I don't love them despite it - but that the fact of being in love is your entry point into not just accepting but welcoming the changes that brings into your life.
When you become the white mother of a black or mixed race child, life does sometimes treat you differently. Some of us were joking on a recent thread that other people think you're a thick slag - and I do think some of that genuinely still exists. But the main thing is that you become a dual heritage family. It kind of alters your own racial identity too. You become responsible for communicating and developing your child's ethnic identity, and so you have to embrace it and make it part of you, too. Not all white mothers accept that responsibility. But if you do, you will find the journey fascinating and far more positive than negative 
One last thing: I have a birth child who shares my ethnic identity, and an adopted child who does not. Do I have a different kind of bond with my biological child? In some ways, yes. She reminds me so much of myself at that age, whereas my adopted child is a very different personality. But that doesn't imply an extra closeness; with both children, I love the things that unite us but also the many ways in which they are different from me. And I love them equally, without a doubt. Some people do seem to set great store on biological connection and physiological similarity with their children - but those people tend not to adopt. Or, for that matter, seek partners from a different ethnic background.
It will be fine, I promise 