Anxiety developing into agorophobia. This is common among anxiety sufferers.
The only thing that will knock it out is medication. I take Zoloft and it works wonders for this type of thing and it's very safe. Don't read those drug reviews on the internet or you'll never take anything. Take it and see how YOU react to it. There are other SSRIs if you want to take one of those instead. They all work basically the same way with similar side effects that are mild in most people. I take Zoloft and I find myself forgetting to take it sometimes which means I'm feeling better.