What would you do if a lady around 30-37 told you that you were useless and would never amount to anything?

She's a well dressed white lady with a brand new Honda SUV and she has a daughter that looks about 12 that is also well dressed and the girl is being rude and both are being very rude to the guy at the counter at Subway. Would it be wrong to curse out the old witch who tried to start a fight and cursed out the subway worker and you?

Yes it would be rude, and may get you in trouble, you don't want trouble.
She's a rude, entitled b*tch who doesn't deserve any of your time, or energy, life will get her one day, walk away, ignore her and leave her alone, she's so not worth your time. Call security if she is causing trouble in the store and leave them to deal with her.