If you can learn how to be more assertive, you are more likely to have choices and to be able to say how you feel and what you think. Assertiveness is not just about getting what you want; it's about getting the best possible outcome for everyone, including you. Getting what you want at someone else's expense is more commonly known as aggressive behaviour.
If you believe in yourself, and believe that not only do you have rights but that you also have responsibilities to others, then you are already well on your way to being assertive.
Assertiveness is too wide an area to explore fully here, but in general an assertive person is less likely to be a stressed person. If you feel that you would benefit from becoming more assertive, getting some assertiveness training could be a rewarding experience and give you skills that will benefit you in lots of other areas of your life.
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.