I know it's wrong, but why am I so nationalistic and racist?

This is an interesting question. Oddly enough, I can relate. When I was growing up I too was picked on by black people. They didn't like the way I talked or made fun of the way I dressed and a whole host of other things and I was very resentful. I grew up not wanting to be around them at all. And their absence was perfectly fine by me. My school was all white, my neighborhood was all white as well as my church until my later years. One day when I was at college, some white people started treating me funny as well and it hit me: Wait! I'm black! I am not being smart or funny. I didn't really know who I was. I didn't learn about black history or any other minority history for that matter really until college. I was mortified. I didn't know where I belonged. I was pretty cold and unsympathetic when I did learn the history because I felt that it had nothing to do with me. I couldn't have been more wrong. By embracing people of all different cultures and walks of life, my world has been so much more full and rich. I said all of this to say that you may not really be racist, just misguided by the way you were treated and yes, I agree. It is a defense mechanism. When you really want to get over it, you will, but not until. It takes time and effort and whether you wish to put in either is really up to you. Just don't let it fester to the point that it gets you into trouble because left unchecked, it will. Good luck.