YouTuber, ex-philosopher 
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Maybe I'm overdoing it and it's not that big a deal but I can't stop thinking about it and he was so helpful and idk I just feel awful. 
just apologize quickly and move on! it probably happens all the time and making too elaborate an apology may just draw more attention to it
So if I do see them again I shouldn't bring it up/apologize, just refer to them as the correct gender? They never said anything to correct me but I was a customer and they were at work and I'm just so mortified.
What I would do, don't make a big deal out of it but just quietly correctly gender them next time when appropriate. "Thank you (correct pronoun)" Don't emphasize, just subtle. You're a good person for worrying about how you affect others 
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