It's true that you need a bit of stress to remain healthy and it's usually only when stress becomes prolonged that it becomes a big problem.
It's vital to manage your stress levels and learn techniques to calm down because a calmer physical state allows the body to heal itself and strengthen the immune system.
I have experienced anxiety attacks and believe me you learn how to calm down quick because high anxiety isn't a pleasant state to be in, to say the least!
Breathing, diet, exercise, sleep and your social life all contribute to your ability to stay calm at any given moment so it's important to pay attention to all these different areas to ensure you have the right balance.
I have found that the Buteyko method of breathing has been helpful and effective in my own case. The main idea is not to over-breathe so with this technique you use shallower breathing to adjust the levels of carbon-dioxide and oxygen in your body.
Diet-wise I have found proper hydration makes a difference in my ability to handle stress, so I make sure to get enough water in every day. Getting the right foods in and staying away from junk food also plays a role in reducing and managing stress for me.
Sleep can easily be overlooked but it's extremely important for your health and your ability to handle and reduce stress. It's important to not only get enough but also proper rest. Your room has to be pitch black with any lights taped over. I also use a white noise generator to mask other sounds which means sudden noises don't disturb my sleep. I also use tinted glasses because I work with computers a lot and this can cause stress and insomnia.
It's always possible to relax a bit more unless you're in a state of complete calm only often found in deep states of meditation. Find out what makes you stress and take steps to manage it. You're worth it!