Honestly the best example I can think of is to secretly record a conversation between you and someone else, and listen to it later. Voice is good, video is better. You'll notice all kinds of things you don't in the moment. Tone, timing, stutters, body language, etc. Then really meditate on who you are and how you want to present to others.
Remember that you can't always change everything... Sometimes you have to accept certain aspects of yourself and work with what you can actually change. Personally I never tried to change anything like that, I'd rather just be authentic, but understand what that actually means and that not everybody will like me.
I wonder if the grocery store could just hire you directly and cut out the middle man? Like just approach the boss and be like. I need a job. You need worker. Gimme a contract and I'll start tomorrow