It can be difficult to fit exercise in, as remember that stress is not a passive thing. It is something you are doing, unconsciously or not, and it takes up a lot of energy and leaves you feeling exhausted. The time when you most need to take exercise is when you are stressed, but this can also be the time when you feel at your most tired and drained.
To make it easier, don't think about the exercise itself, but focus on how good you'll feel after doing it. This will help to motivate you to go out for a walk during your lunch hour, or to go for a run when you get home from work. Once you have been doing it for a few weeks, it will become a habit, and you won't even need to think about it anymore. Soon you'll be fitting more and more exercise into 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.