How am I supposed to move on?

My ex-boyfriend (21M) crossed my (20F) boundaries so many times that I eventually had to call it quits but he was my first love and first relationship that I cared about. It feels unfair to me because I wasn’t his first love, he’s keeping the apartment we got together, and even keeping our cat and record player. He’s moving on much faster than me even though he’s the one that fucked up. How do I become strangers with someone that I loved so much again?