The most effective treatment for shyness I've found, was by far retail customer service.
Anyone can get these jobs, probably even somewhere fun part-time. The bar is so low for this type of job they will take almost anyone. It's very easy to distill a business interaction down to a short, easy script. Then repeat that dozens of times a day for a year and suddenly you're an expert at small talk with every kind of person.
Or do some charity collecting. I mean with a tin in the street, not the UK thing we have at the moment of hordes of paid street spammers trying to get contact details.
Similar situation to shop work, but only nicer people will approach you, the ones who know the charity and want to give something. People like to tell their stories as well, which can help.
Anyone can get these jobs, probably even somewhere fun part-time. The bar is so low for this type of job they will take almost anyone. It's very easy to distill a business interaction down to a short, easy script. Then repeat that dozens of times a day for a year and suddenly you're an expert at small talk with every kind of person.
It really worked for me, highly recommended.