I assume that you did such a good job that they thought you had adult help. I am sorry that happened. It unfortunate that so many kids cheat that on school assignments and projects and don’t do the work themselves - That causes teachers to be hyper vigilant. Obviously they made a mistake in your case. It’s too late now, but at the time you could have protested the judgement and showed them your work that led to the project. The real point of a science fair project is that you learn about the subject and not the credit. I certainly hope you enjoyed doing the project and learned things that help you. In the long run you are better for having done a good job than if you had dumbed it down. Life is long, it could still pay dividends for you in your career or interests as a step on the way.

Let it go. It's not like being bitter when the judge dinged me for having the hypothesis wrong (when, in fact, my hypothesis was correct.) My bitterness only grew when I learned more about scientific method where the hypothesis can be supported or not as still be good science! I mean, some random person's bitterness.