Actions to take: Think of your stress like any other workflow issue, not as a personal problem. If you are at or near your stress limit: 1) Don't take on new work without reducing time spent on other tasks. 2) Begin reducing your workload. 3) Temporarily ignore other stress-inducing activities. 4) Take a full week vacation as soon as is feasible.
We live in an extremely stressful time. There are a lot of great resources for coping with stress. I encourage you to seek them out. This post is about reducing your stress through workload management. The following tips, while difficult to achieve in some work environments, apply to virtually any workplace at virtually any level in the organization.
Discuss new work with the assumption that you already have a full workload
Every time you are assigned new work, have a conversation with your boss about priorities. Ask them what work should be deferred/stopped in order to accommodate the new task. Approach this discussion as if it is understood that you already have a full workload.
The default assumption in a work environment is that people can take on extra work while maintaining all of the duties they currently have. This is largely true. We get better at tasks as time goes on. You might be able to do twice as much in your 3rd or 4th year as you were able to accomplish in your 1st.
You need to make it clear that this is not case for you right now. Have estimates of how much time various projects and tasks are taking. Be confident and relaxed, but firm in your explanation that you cannot add new work until you make room by removing something else. Don't approach this as a "you" issue. Approach it the same way you would for, say, production capacity on an assembly line. You could overclock the machinery and destroy it, or you can keep production within normal parameters. It is the same thing with you: "Here is my available output. What should we apply it to?"
Confidently begin to reduce your workload
We just said you would approach conversations about new work as if you are at full capacity. If this post is speaking to you, you are probably beyond full capacity. We can get tricked into believing that 10 hour workdays are fine and typical.
When projects wrap, don't take on new ones. When you figure out how to shave an hour from the time spent creating that monthly report, don't fill it with something else. Have you been the chair of some committee for the past 2 years? Maybe it's time to let someone else get experience with it. Start reducing your workload as directly or as subtly as you think appropriate for your work situation.
Regardless of how you do it, actively assert the idea that you are indeed overworked, even if only to yourself. In the absence of information, bosses assume that your status is "fine." When we are stressed, we don't have the free mental energy to engage in self-analysis. Without that mental energy, we rely on others' cues about the world to guide us—we just don't have the capacity to guide ourselves. Our stress causes us to take cues from the boss ("If the boss thinks this is fine, it must be fine"), which means more work, which means more stress, which means even less ability to speak up about the fact that we are overworked.
You have to break the cycle. Be positive, practical, and matter-of-fact about it. Again, this is not a "you" problem. It is just a capacity issue that needs correcting.
Take a break from as many energy-draining activities as possible, even if it means letting things go for a while.
Time is finite resource. Start thinking of mental energy as a finite resources too.
Our tank of mental energy gets drained from many places. Under normal circumstances, we barely notice most of them. We have perhaps two or three main sources of stress. We recognize those as the things that eat up our mental energy and monitor them accordingly.
When you are at your stress limit, all of the tiny sources of stress become important. Anything that uses our energy can become the thing that puts us over the edge. Take a break from small stressors. That might mean letting the house go uncleaned for a while, skipping your weekly phone call with your well-meaning-but-judgmental relative, and uninstalling your social media apps.
Taking a break is the easy part. The hard part is using that break effectively. You'll only feel worse about yourself if you pause a diet only to spend more energy reading about world issues. We are temporarily cutting out little sources of stress to gain the energy to do the things described elsewhere in this post. Don't squander it.
As soon as you can manage, take a full week vacation, Monday through Friday
A full week vacation does much more to reset your stress than anything shorter. It is also significantly better than taking mid-week-to-mid-week vacations. Part of this is getting more days in a row with weekends on each end.
A subtle but bigger part is that you are fully pulled out of the weekly workflow. When you take vacations, say, starting and ending on Wednesday, you still have to be engaged in both weeks' work. You essentially do a full week's work on Monday and Tuesday before leaving the office, then a full week's work on Thursday and Friday after returning. Even though that would be 6 days of vacation time, it does less to recharge you than a full Monday-Friday vacation.
Your return is an excellent time to start deferring work. For the first two days you are back, it is easy to take on absolutely no new work: "I'm still catching up from time off. Can we touch base about that in a week or two?" If it was important, it will get shunted to someone else. If it was not, odds are 50/50 that it will be forgotten about for the time being.
Conclusion: Do fewer things better
Somewhat paradoxically, the more you place your own wellbeing ahead of you work, the better your work will be. It can be easy to believe that you need to take on as much work as you possibly can in order to succeed. The work needs to get done after all, right?
No. It doesn't. There will always be more things worth doing than time (or energy) to do them. Chose what is most important, and ensure that you have the capacity necessary to do it well.