Hi there, yeah that sounds quite stressful for you but I wouldn't worry.
Your work wasn't actually 'published' by the YouTuber or your comment (i.e. you didnt put any copyright claims on it) so I don't think you've inadvertently plagiarised yourself.
Is it that your username om youtube isn't you're real name so the exam board wouldn't know you're copying your own work if they even did see the video? If so, I don't think exam boards have software that run plagiarism checks to the extent of checking youtube videos.
Have a word with your teacher and see what they say, worse comes to worst you have to tell the exam board but it's highly unlikely you would get penalised.
Pepole copy ideas all the time - think about people who copied word for word what their English Lit revision textbook said in the exam and claimed it was their own idea, or people who learn word for word parts of a bio textbook.