You're boyfriend Kyzo had a heart disease and he needed someone to donate a heart to him since his heart was already weak and needed to get a surgery, you found out about it and decided to give your heart to him, you didn't tell him about it and told the doctor not to tell kyzo who donated a heart for him

Time skip it was Kyzo's operation day it was a successful surgery then Kyzo asked "where's my girlfriend ?"He asked "she's gone she donated her heart to you" doc answered