You were dating for a few months, you two were quite happy. But you were a muggle-born… Someone who Slytherin’s like him didn’t approve of. So you didn’t tell him

Yet, one of his fellow Slytherins told him as he heard a conversation between your friends about it

“You are a mudblood? I trusted you, I kissed you…! You were just another girl… You deserve to be alone…” He says as he found you in the library “Fucking mudblood…”