Who does the shopping? Does she go to the docs or dentist? Is there even an hour window for you to get out without her there? Ugg, doesn't sound easy, but even if you have to go in the middle of the night, then that's what it may take. The other thing that may have to happen is to get the help from the woman's shelter, perhaps they can be with you when you leave? Not sure how it works, but at least if you had someone like that you can have help. The police may help too, as to keep you prisoner is illegal, let alone abuse. I know it sounds dramatic, but you're in an extreme situation, so sometimes extreme things need to happen to get away.
Am sorry that you're going through this - you will find a way, am sure of it.