The customer is not always right. There are some customers who really are just dicks, and your employer will agree with you. You don't have to worship the ground your customers walk on. Be exceptional to the good ones. Deal with the bad ones. Just smile and be polite to the rest. Everybody will say "Go out of your way to be great to the customers!"
My advice is "Don't".
There is just not enough energy in the world to be nice to every customer.
If we found some miraculous new renewable energy source that could power every electrical device on the entire planet and fuel every car for the next gajillion years, we would still not have the energy required for a human being to put that much effort into a retail position.
Don't even attempt it.
You will ruin yourself.
There's nothing that'll make you hate your job more than having to smile at an asshole and go out of your way to help them.
If you want to better deal and cope with your job, forget about your job. You're not your job. I know it's taking up all your time right now, and it feels like there's no space for anything else. I know you feel like you don't get to be yourself and that you're not enjoying yourself. It's all the job's fault, you're miserable when you're there. There are unrealistic demands. It's stressful and time consuming, draining and exhausting.
Forget about that. Make the most of the time you have outside of work. Enjoy yourself. Relax. Do things that make you happy. Make sure you go to bed smiling. Have a breakfast that puts you in a great mood. Listen to happy music. Talk to people that make you feel good.
Make the most of your free time and your job will get a whole lot easier. You get to go home after your shift ends, don't you? You have job security, right? Being happy and healthy is far more important than being an amazing salesperson. They're probably not even looking for that, they're just looking for somebody who can handle the position, and you can. You can handle the fuck out of that shit. You just have to calm down. Decompress. Unzip. Unwind. Sprawl. If you find your job stressful, it shows.
I bet your job isn't even hard and that you just feel overwhelmed. You can get on top of it. Do what you have to, not what you can, don't overexert yourself on something as unimportant as making sure that the obnxoious fat guy who spawls all over you when he talks has a comprehensive answer to one of his questions just so he can ask a male salesperson instead.
Tell that asshole that you're busy. You have more important things to worry about.
More importance on your wellbeing, less important on your job.
In ten years, it won't even matter whether or not you were an amazing technology consultant for Staples. You won't have to work in retail anymore. You could put all the energy you have into this job, and just a few years down the line, there will have been no real benefit from it. You'll still remember it as a stressful and difficult time, though, if you keep exhausting yourself. You think this is bad, just wait for Grad School to kick your ass. From one stressful environment to another, right? IF you keep exhausting yourself.
So don't. This time next year, it won't matter.
You don't have to do this forever. You only have to do it now.
Having a nice stress/bullshit free day is the best way to handle this, I am sure. Do a good job. But don't slowly kill yourself.