I invented Nihilism when I was 8 or 9. My parents told me to shut up talking nonsense. I remember reading about it as a teenager in Reader's Digest and thinking someone had ripped my ideas off - I came up with that by myself years back😆
I also invented the sanitary towel around 5 or 6- my mum just used tampons so i didn't know what pads were. I overheard her moaning about getting stains on her knickers so I made a wad of loo roll with improvised sticky back plastic (looped black electrical tape) and presented it to her. Her and her friends found it hilarious...I was most disgruntled.
I had the idea for the kindle too as a kid in a cold house who wanted to read but keep hands snuggly and warm. But mine was wall mounted and you would blink to turn pages so I still might take that one to Dragon's Den.
Have you ever sworn you've invented something that already exists?