Is it normal to only be able to focus on one thing

I have had to deal with this my WHOLE life, and I can't understand if it's normal or if there is really something wrong with me. In a conversation an my friends or parents are making plans to do something I CAN'T move on to another planning of something until the first planing is completely done. When they don't listen and keep going on and on with the conversation I try to block them out of my mind and stop listening but it doesn't work! My head will start to pound or it'll feel as if a million words or people or something is bouncing around in my head. Rarely I'll start to rock back and forth and my eyes will be locked somewhere on the floor. I haven't been tested for anything like ADHD or ADD but I'm getting very worried about this.