How can I stop regretting this past mistake?

ust a little bit of background for the question. a few months ago a girl come into my job three diff times she was buying something for a construction site she came in two other times but i did not have the balls to asked her for her number.

i was into this girl i never asked her for her number at any of the three times she was in my store i'm
regretting this mistake.
How can I stop regretting this past mistake?
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