It's important to realise that at any one point, you do have choices, even if it feels as if you don't. For example, if you are feeling under pressure at work, due to large amounts of overtime etc, you may feel that you have no choice but to resign. This is still a choice, even if it would be better if you didn't resign. It's empowering to think about what choices you have. To make it easier, imagine yourself in the future, looking back at yourself now. Give yourself some perspective.
Or you could imagine a very assertive person that you know. What would he or she do in the same situation? What might you do if you were a different person? Actually, you have the same choices. You might also want to discuss it with a friend. What would they do? Speak to someone who isn't personally involved. Brainstorm a whole load of different choices, easy, hard, silly, extreme choices - put them all down together to get you thinking in this way. This may not solve your problem, but it is the first step to creating choices, even if you don't act on them. Start to act like a person with choices, and you'll already start to feel better.
Stress is a medical condition and it can have a serious effect on your health. If you think you are suffering from stress it is crucial that you consult a medical professional.