Complex sentence: Before he met Hibiki, Shirotsuki was in a relationship with a woman named Yui, but after he started to get serious about magic, his memory started to fade until ultimately he had forgotten about Yui entirely.
Split into simpler sentences: Before he met Hibiki, Shirotsuki was in a relationship with a woman named Yui. But after he started to get serious about magic, his memory started to fade until he had forgotten about Yui entirely.