I find this kinda fascinating because if you've thought this your entire life, you would have had to have read every intended plural "you" as singular and there would be nothing to inform you otherwise without very explicit context.

For the record, every 2nd person pronoun in the preceding paragraph can be singular or plural and still be grammatically correct while remaining socially correct for my speaking to an audience of unknown size. "You all" and "you guys" are slang phrases that don't appear in formal writing.