Hey man. I’m 17 and work a cashier position at Walmart. (Willingly) and I found that it’s actually helped my speech improve quite a bit. I also read aloud about an hour a day, which helps as well. It’s absolutely horrific that you were thrust into this experience without being prepared for it. But I don’t think that you should attempt to avoid positions like that forever. I prepared for a few months and took the job willingly, and I found that I felt much more confident and I was even more fluent when I felt that I was the one in control, that I was the one that chose this. If you can just stock shelves somewhere for now, then do that, and try to keep practicing your speech on your own if you have the time to. The best way I could think of when it comes to not being forced into a cashier position is just to have a heart-to-heart with your manager/boss and explaining your circumstances. Best of luck to you dude.