How to not hate people while working retail/customer service?

Been in retail/CS of some form for well over a decade now; from regular associate, to supervisor, mgr, ASM, back to associate...I've always just tried to remind myself that people will be people, but for some reason lately it's REALLY taking a toll on my mental. The recent (past maybe 2 years) influx of just absolute turds with no regard for anyone but themselves have started to make me really...bitter?

How do you personally not let yourself get that way, or even if you do, how do you hide it?! Like I had a customer earlier insisting that a $20 bill would pay for her $120 order, and only after about 9 times of explaining it in as many different ways as I could, did she understand that I needed $100 more to cash her order out. Had a couple behind her who kept audibly making fun of me for being irritated even though I wasn't trying to show it. I constantly catch customers making fun of me and just have to ignore it and remind my face to relax.

Idk. Frustrated, annoyed, wits end. Venting. Anyone else?