I agree with Christine.
You sound like you are pretty intelligent (from the way you write). That could explain why some people think you have it and some don't. If you have ADD, the intelligence will mask it until you get to the point that the subject matter is so difficult, it becomes more obvious. For kids with good intelligence, Algebra is usually what does it.
Anyway, check out this site as there is lots of good info. The site is totallyadd.com and is from an award wining PBS special. Thus it is broken down into a lot of short videos. If it hits home. Have your parents take a look at it. You will get a lot further along in the testing process, if your parents are behind you. Hope this helps. best wishes!!!
You cannot diagnose yourself or be diagnosed over the Internet. You need to see a specialist for some tests. You may want to start with a counselor at your school. Most schools in the US have a Special Needs teacher and often they are qualified to diagnose learning disabilities (such as ADHD).
My stepdaughter has mild ADHD and it was diagnosed through her school. She now functions VERY well thanks to a few modifications.