It is very hard to pinpoint a single moment. Rather I would like to list a few books that I feel showed me that mathematics can be beautiful and interesting.

1. Measurement by Paul Lockhart

2. An Infinite Descent into Pure Mathematics by Clive Newstead (available @ https://infinitedescent.xyz/). It taught me basic discrete math and proof writing.

3. Apostol's Calculus Vol 1. It is just so beautiful.