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Try to breathe, play some music on your phone if that’s an option. Music at work at all can help.. I usually sing to myself at work.
I had to fight with myself to stay at my first job, cashiering at a DG. It involved stocking too. I still have times i need to take a time out from what I’m doing and sit for a moment away from the work.
I used to go outside at break but it was warmer and daytime so that was an option. Part time made it easier too. I like being outside for break at this job in the summer, but nowadays it’s too cold.

Jobs are different. Not liking this job, or “handling” it doesnt mean there’s not a job you’d be more equipped for or likely to enjoy. In my experience muscle soreness can go away or it can be made worse by the job, if it wears on you too much it’s not a sign you can’t hold down a job, just that your body isn’t used to that kind of strain. Work is also easier with coworkers you like..
You’re trying to juggle school and a job, it’s a lot. Most people have trouble balancing one or the other, let alone both. Again, just try to breathe.