My anxiety isn't as bad as yours, but for the first month of my retail job, I just had to leave very early in the morning so I'd have enough time to drive sloooowly to work and to sit and breathe for a while before I went in. After about a month, the anxiety lessened and I had a much easier time. It was still a really stressful and exhausting job, but I did manage to work a full year at Macy's, and my managers were very forgiving of my occasional lateness.

My REAL problem was that the job made me really tired, and I had a big fear of driving at the time. So I'd be totally worn out at the end of the day, sometimes shaking from all the stress, and then I'd have to DRIVE HOME. O.o Not fun, and this lasted way longer than my pre-work jitters. Eventually I started bringing a CD player, a book, and my Nintendo with me, and after work I had a three step system:
1) Listen to music until I'm calm enough to play video games
2) Play video games until I'm calm enough to read
3) Read until I'm calm enough to drive

It worked REALLY WELL. So maybe you could take some "security blankets" like that with you? Like if drawing or listening to music makes you feel calmer, you could do that before going in? I don't know. Like I said your anxiety is a lot worse than mine. But it couldn't hurt?

EDIT: And I just thought of something! I also actually WROTE DOWN a sort of "script" of what to say to customers. Like, "If the customer says [whatever], I will say [something appropriate]." It was so fake, but a big part of my social anxiety is worrying that I won't know what to say, so that helped a lot! And believe it or not, I actually got lots of praise for my excellent customer service!! lol