I'd get on
www.odesk.com and
www.elance.com as a designer. There are all sorts of people on there looking for designers to create website layouts, advertising material, etc. for them. You can set your own rate and let people find you, or you can actively search for the jobs that people are posting for design work.
As others have said, you're selling yourself short. 10 years experience working on Photoshop means you know how to use it when most people don't. Simple as that. There will always be more to learn, but you can do that while working on new projects.
I can pretty much guarantee you that you'd be making a heck of a lot more working on your own as a sub-contractor than you are based on what you've said here. I wouldn't just quit the job and start it immediately, but get yourself on those platforms, get a couple of side jobs, and I'd bet you'll quickly feel a lot more comfortable telling them to screw off.
Then, who knows, they'll probably realize how much they need you at that point and fight to keep you.