I have worked on this a lot and would say I'm calm and laid back the majority of the time, it's certainly been commented on by friends but I haven't always been this way.
My mum is a highly anxious, stressed worrier. A lot of her anxiety and worrying rubbed off onto me throughout my upbringing and I don't want to do that to my own DDs. In my case, it was the realisation that all of mum's worrying and catastrophising hasn't bettered her life in any way, it hasn't improved outcomes, all it has meant is that she's not a pleasant person to spend time with. That was quite a hard pill for me to swallow but it's true. She's negative and her negativity rubs off onto people around her. I have a friend that is the same, anxious and always catastrophising and going down "what if" rabbit holes and she's exhausting to spend time with. It must be exhausting living in her head and it doesn't seem to improve her life at all.
I don't want that for myself so I try really hard to create some distance in my own head. With work for example if I find myself stressing I will step back and remind myself that it's not that deep, it won't matter this time next year, I work in an office job..... it's not a life or death situation.
I don't watch the news. I will listen to the headlines on the radio so I know what's going on but that's it.
I have stepped back from negative people and spend more time with people that make me happy. I also really curate my social media so I do have Instagram but all of the accounts I follow are about slow living, homesteading, gardening, cooking etc and it's therefore a pleasant place to be rather than stressful.
I try to limit stress as much as is in my control and live as simply as possible. We don't have pets for example, because they'd be just one more thing to have to deal with. I limit how many clubs DDs do so I'm not constantly ferrying them round to different activities. We get outside for walks and like gardening and growing our own food. I do yoga or go on a walk for exercise as I find both a mindful activity. I try to eat a whole food diet as much as possible and limit the ultra processed junk.
I believe that in many circumstances, happiness is a choice and I choose to be happy. We have had hard times, been through bereavement and redundancy etc but those things are part of life and we've survived them and found happiness again so we'll survive the next bad thing that comes our way.