Most of my work is in retail and I assure you, we don't remember you out of the hundreds of customers we get each day.
The people we remember are the ones who make a big deal about not being able to return or exchange already-used products, or who lose their s**t because one product scanned up at a different price than what was on the sticker.
Also, most of the talking we do with customers is part of our training. We're not saying hi or have a nice day to you because we care, we're doing it because it's part of a script that we have to follow. If you slip up in replying to that, we don't notice, because a lot of NT people do as well.
Honestly, interactions with staff in grocery stores and shopping malls aren't really social interactions. I'm pretty sure a lot of the staff working in those places would love it if we didn't have to pretend to care about our customers! Please don't stress about something like that.