My approach to this has been reasonably effective but is far from perfect.
Firstly, if something that causes me stress is avoidable, I simply avoid it. For me, that has included cutting ties with certain people who were excessively dramatic, ceasing ALL social media use, avoiding mass media ~99% of the time, avoiding going to parts of the city that I don’t like, and otherwise decline to engage. That includes arguing with people online…
Next, if something must be dealt with (but not necessarily by me, then I will (in this order) automate or outsource/delegate the work to someone else. I don’t stress about bills when they just come out of my bank account each month. I review the payments to make sure nothing is amiss, but for the most part I just leave it be. For things I can’t automate, I will have someone else do as much as I can. The biggest example of this is taxes… I can’t stand even the thought of that, so I very happily pay accountants to just get it done. A previous example of this was neither my wife nor I enjoy cleaning the house, so when our daughter was very little and needed more attention, we hired a cleaning service to clean our house for us every week. That wasn’t so much relieving stress, it just freed up some time and was one less thing to think about.
The remainder is split into tasks that must be done and situations that must be dealt with. For the tasks, I find having a system or a routine to handle them makes them easier to do… This does not always work; just a couple of weeks ago I had a tax situation that only I could resolve, and I put it off for quite a while… so this part of my “process” is where there are still flaws. Anything I can do to make something simpler and take the stress off my plate, I’ll do. Occasionally when enough “things” back up, I need to just psych myself up and power through them… this may be making some unpleasant phone calls, or otherwise doing something difficult enough that I’d put it off… I’m still working on improving this part of the process.
Now that leaves general “life stuff” that causes me stress, and which isn’t avoidable… the health of my aging parents (and other family members) is one thing, and to be perfectly honest… I know intellectually that even if I do everything I can to help, provide useful information, buy supplements, whatever… I can’t make anyone actually take actions they don’t (yet) want to take.
So… finally getting to the meat of what I assume you’re asking about… I study Stoicism, Buddhism, and try to understand what it is about the situations that make me feel stress. One of the more helpful books I’ve read recently on this topic is Loving What Is, by Byron Katie. My main takeaway is that I have a mismatch on “what is” vs “what ought to be”.
I’m still working on fixing that last bit. All of the work to avoid/automate/delegate/systematize as much as I can frees up my mental capacity to deal with what’s left. It makes the rest of it much more manageable.