I have some speculative thoughts on the subject.
First, research on heart disease shows that the most potent predictor is living at the bottom of a hierarchy (see work by Marmot, 'the Whitehall Studies'). You'd think the effect is surely an error attributable to a confounder, but none have been found. Access to health care, smoking, diet, exercise, poverty . . . nope. And the measures can be quite gross--like societal level measures of inequality such as the Gini index--and big effects are evident. For example, different post codes in Chicago can have a near 20 year life expectancy difference.
Other work by Sapolsky on stress response and hierarchies in baboons shows the same effects. Baboons at the bottom of the hierarchy are subject to capricious stressors, and they develop abdominal fat and the same poor metabolic health markers that we have. His work also shows that a re-structuring of the hierarchy so it becomes less harsh decreases rates of aggression and the health markers improve.
I guess this is a long way of saying that if you are unable to restructure your social environment, moving to a new one can be good advice.
Second, absent the ability to make such changes, there are multiple things we can do to affect how we respond to stress. Off the top of my head, and to reiterate/add to what has been stated:
1. Meditation has been shown effective. A free app like insight timer can have big effects--one study I recall showed a lengthening of telomeres;
2. Sleep is very important for maintaining health. If you are having trouble sleeping, look into strategies to improve it (e.g., get rid of bright light, sleep early and on a fixed schedule; use your bed for sleep only; possibly consider melatonin etc.);
3. Exercise is important for how you react to stress. Figure out some way of pushing yourself to the point of getting a good sweat on 3 or more times per week. e.g., run, bike, climb stairs, hike up a mountain . . .
4. Diet is important. Eat real food, fresh veges, green leafy stuff, some fruit, and if you eat meat eat the stuff that ate a natural diet (e.g., grass fed beef to get the omega 3 to 6 ratio in your favor; wild caught fish). Avoid processed shit at all costs.
5. If you are stressed, it is likely that you are both depressed and anxious (I forget the exact correlation, but it's high and generally if you have one, you have the other). There's good evidence that cognitive behavioral therapy can improve both, and you can even self-administer (e.g., see David Burns, the feeling good handbook). I'd be inclined to see a clinical psychologist or good psychiatrist if you are past self-help, of course.
6. Social support is important. If you have an ally or two who you are sure you can confide in, that may help. It may also help to do other things that give you more control over your life. Do you really need to say 'yes' all of the time? Do you really need to work those extra hours? Answer that email at 2am?
I'm sure there's more, but it seems to me that for those of us stuck in a situation, there are multiple and simple things we can do to improve our reactions to stress.