我只是个备胎而已
[wŏ zhĭ shì gè bèi tāi ér yĭ]
Translating as 'I'm just a spare tire', it metaphorically refers to being a backup partner or friend. It reflects self-deprecation that might imply sadness or humor. Such a user could be acknowledging their place in others' lives or perhaps expressing dissatisfaction over it.