You’re looking for counter-arguments for Solipsism. Wikipedia’s definition:

Solipsism is the philosophical idea that only one’s mind is sure to exist.

Further reading: [iep.utm.edu/solipsis/](Solipsism and the Problem of Other Minds)

Thread with some counter-arguments: https://philosophy.stackexchange.com/questions/89321/what-are-the-best-arguments-against-solipsism

The one I personally chose for myself is the pragmatic one: Believing reality is a fantasy doesn’t actually changes your experience of it.