What major discovery after the 1920s makes you think I'm wrong? As a person who spent every waking moment concentrating on math from ages 12 to 19, I'm pretty confident of my answer. The only counterexample I could think of is the FFT algorithm in 1965. The inner workings of the DFT were discovered in 1822 though.
I can clarify my stance with this statement:
If you think you've discovered something new and unpublished about mathematics, you're probably wrong, there's a good 95% chance it was already discovered, published and perfected before the 1920s.
1. Research and find as many prime number theorems as possible to make sure his isn't a reformulation of any of them.
2. Publish, as DavidP suggested.