I answered this one before but I didn't see the therapist update.
Again, you're going to have to break out of your comfort zone to do this. If it's something you really want, you need to take the first step. However, that doesn't have to be a huge step. Baby steps can work over time. You just need to make up your mind that this is what you want. I clipped my first answer so you could read it again. Take the small steps and you should do fine.
I was like that as a kid. You couldn't get me to talk in front of the class, to strangers, anyone I didn't know.
You have a unique opportunity because you work in retail. I give this piece of advice to anyone who asks about overcoming shyness. You can take it even further because you deal with customers everyday.
Here's what I normally tell people. When you go to the store and you're at the check-out, start a quick simple conversation with the clerk. Start with "Hi, How is your day going?". Let them answer. That's it. Keep repeating this until you feel confident enough to ask a second question: "That's good. Has it been busy today?" Again, build on these small steps until you feel comfortable to continue the conversation.
For you, you can do this all day long. Except, you can turn the tables and ask the customer simple questions: "How is your day going?" "What brings you here today" "Is it still nice outside? I've been stuck in here". Do you see how these small quick questions can help you build a) confidence with talking to strangers, and b) build rapport with your potential customers.
Another way to "break the ice" is to ask them something personal: "Hey, I love that shirt. Where did you get it?" or "Now that's a lovely ring. How long have you had it".
The point is to build your confidence slowly. Don't rush it. Just get to the point where you feel comfortable just asking 1 question. Then build on that. It will take some time and you'll have good days and bad. But as long as you're moving forward, you should be able to overcome your shyness enough to move forward with your life (e.g. go to college).
Best of luck.
There's a book called "Public Speaking Without Panic" that has tips like this and others to help you build confidence in your public speaking ability. Go to Amazon and type it in or click the links below.