I have found that always having an alternate job waiting almost eliminates most of my stress and I am than far more capable of speaking up for myself. If you can manage to set up something like this I'm quite sure it would help you too.

But as you already know it comes down to just standing up for yourself and your rights. Which I know is easier said than done, for me it was nearly impossible without a backup job to rely on. I do eventually get tired of not sticking up for myself and the pressure builds until I blow my top and start yelling at my boss which of course usually ends up with me losing my job.

I rarely work now but when I do it is still as an independent online contractor and I lay out the ground rules before I even accept the work. I don't ever want the misunderstanding from my clients (or boss) again. This has greatly helped me to never give them more than they are paying for and I can finish the job at my leisure (within reasonable limits).

Good luck with your email, let us know what happens!