Sworn Enemy or Lover?

Lu Xili had cancer and only had three months to live. He unexpectedly ran into his first love Chen Junge at the hospital. Lu thought the meeting was fate, but he saw that Chen already had a boyfriend. Three months after his sad death, Lu Xili was reborn into the body of his first love's boyfriend. That was when he discovered that they weren't a couple at all, but they were actually bitter rivals... the journey to get together with his love will be longer than he thought!

(Source: Kuaikan, translated)