只有大姨妈不离不弃
[zhĭ yŏu dà yí mā bù lí bù qì]
'Only Aunt Flo (a euphemism for menstruation) sticks by' is an idiom used to describe when everyone else has left and one only has themselves or some regular occurrence they have to endure (like monthly menstrual periods). This implies a sarcastic view on loneliness or feeling abandoned except by something no one really enjoys - similar to 'Aunt Flo' in English context which refers to a period.