Hi, customers at work can seem really horrible, I've worked in telesales before so believe me I know

as long as you know that you're doing the best you can then it doesn't matter how rude they get they are just wrong, it's just realising that at the time that's easier said than done.
I work in a bar now where most of the customers seem pleasant but there are unpleasant people as well, I actually find myself antagonising customers at work even if I don't know them but that's something I have to work on personally.
As much as I'd rather avoid people I think the exposure will probably help without even knowing it and it could do the same for you too.
With that said I haven't walked in your shoes but yeah, you aren't alone in this