自以为白天鹅的丑小鸭
[zì yĭ wéi bái tiān é de chŏu xiăo yā]
'An Ugly Duckling Thinking It’s a White Swan.' Referencing Andersen's fable where a young bird thought badly of itself until realizing its own true nature. The netizen uses this idiomatically for self-deprecation regarding perceived flaws versus imagined ideals which usually turn out falsely high opinions of oneself.