The Infinity stones have existed since the origin of the Universe.
Since the Mind stone is so powerful how can the Scarlet Witch break it in Avengers: Infinity War?
Because her powers are the same energy as the Mind Stone.
As seen in Age of Ultron, Wanda and Pietro were given their powers by von Strucker, using Loki's Scepter (which contained the Mind Stone). Because of this, Wanda is the only person who could destroy the stone.
The relevant quote from Infinity War comes from Vision:
I've been giving a good deal of thought to this entity in my head, about its nature, but also its composition. I think if it were exposed to a sufficiently powerful energy source, something very similar to its own signature perhaps, its molecular integrity could fail.
You know how it's possible to shatter a wine glass by making it vibrate at exactly the right frequency? That's the exact same principle Wanda used to shatter the Mind Stone. Yes, it may be an immensely powerful crystal, formed in the very heart of the Big Bang... but it's still a crystal. It's just as vulnerable to resonant frequencies as glass.
I believe Vision explains this when he first brings up the idea of Wanda shattering the Mind Stone.
I've been giving a good deal of thought to this entity in my head about its nature. But also, its composition.
I think if it were exposed to a sufficiently powerful energy source, something, very similar to its own signature, perhaps...
Its molecular integrity could fail.