To ask how you manage to reduce stress in mid life

I find Sertraline works a treat

This woman has said everything I would have told you.
She is talking sense.
Lists are your friend.
Delegation is key.

Write a list today. Delegate & watch the tasks melt away.

As PP suggestions.

I have also found for me its how I react/deal with stress. Anything I can turn into stress, and unfortunately we can't predict what stresses will suddenly come upon us. (Sickness, sudden financial issues, losing jobs ect)

Again that will be personal for you. I use meditation to centre myself and trying to change perspective or even it out..rather then thinking how something can go wrong, also think how it can go right and trying to spin a positive out of it (usually thin air 😂) I find it's a lot about changing how your mind processes things, so it's trying to question the instant reactive "ahh this problem, that problem, then got to do this, this, this" to this issue I will think about first for a few mins and think about that one tomorrow when it is relevant, let's then finish with thinking about the restaurant meal I'm looking forward to and what I will order.

Make sure you always have time in the evening to decompress before bed. (I call it Zen mode) I hide in the shower so no one can call for me 😂 hot water, nice smells, fluffy warm socks after.

I honestly feel that a lot of this is about me looking after my body and mind rather than actually anything material about the kids/home/family life
This isn't for everyone but would something like the '5 minute journal ' work for you
https://www.intelligentchange.com/products/the-five-minute-journal?variant=40396118949961&currency=GBP&utm_source=google&utm_medium=cpc&utm_campaign=-&adgroupid=&utm_content=&utm_term=&gad_source=1&gclid=Cj0KCQjwrKu2BhDkARIsAD7GBoskIvHXHPAhql5a5KVmTxhJzevyyrq7tcIJro5TVNWx5MCPTli8rcYaAvq8EALw_wcB
It can help gather your thoughts, or maybe some meditation before you start work or in your break?
Day walks have helped me a lot. Even if it's walking for 20 mins around the estate makes me feel better.

It gets easier as they get older as PP said.

From your description I am doubtful your DP is pulling his weight. Divvy up whole areas - you do medical, he does dental, he does packed lunches you do sports kids. He does the car and insurance you do household bills etc, he does laundry, you do shopping and breakfast etc etc That way you can ditch entire areas from your brain.

Simplify and automate as many systems as you can. Cook meals (or a local service doing the same) half the week, assembly meals the rest, maybe one batch cook at the weekend. Automate all bills and shopping. Have days for laundry and ordering food, have a meal plan, a system so making packed lunches and getting together uniforms all have a time they get done so you don’t have to think about it much m. Put it all on a wall chart. Make sure it’s evenly divided. Have a shared calendar and a shared notes system.

Drop your standards if needed, he may not do laundry as you would but as long as it gets done it’s fine.

Tackle looking after yourself bit by bit - you need enough sleep, to eat properly, to get some exercise and to get some time off alone and with friends. He needs the same. It’s important you get equal time off, so start with one thing first each. You will gradually find out if you like yoga or boxing, just try stuff - you won’t have tons of time now, but you need to get your head out of the grind regularly - can be at home while he takes charge as well as going out.

Gradually train your kids to do their bit, starting with putting dishes in the dishwasher or something

It will get better

I did ( and still do) what @purpledagger suggests, lists and plan.
How me and dh used to operate was Sunday morning I would meal plan for the week and I'd write out a shopping list. Dh used to take the kids to the big Tesco and get the shopping done while I prioritised what house jobs needed doing. In the afternoon we would either go out with the children together or one would go and the other had free time. Dh always chose to give me as much free time as I needed.
Have a planner on view and update with what's going on, eg swimming lessons and check it off the evening before so bags can be packed.
Get the children to do their share of chores. Don't make a big deal about it but get them to make the bed, put toys away, plates in sink or dish washer after they've been scrapped clean, uniforms off as soon as they come home from school etc. It's not you V everyone, you are a team that should have each other's backs.

While the childrenare playing or what ever do a 15/20 minute yoga pr pilates work out through your tv from youtube.
It will get easier but lists, plans and charts are your saviours at the moment.

Firstly, remember that things will get easier, your children are young, but they won't be forever. Mine are both in secondary school, but i do remember how difficult those early years are. i also couldn't afford to outsource work or reduce my hours.

what helped me, was getting organised with the things i could control, which was the everyday stuff.

i wrote lists of literally everything we needed to do and crossed off things as i achieved them. once it's written down, i realised much of it was achievable, but the stress was relying on my memory. i also had a calendar on the wall, so i could keep track of the upcoming month.

i automated whatever i could, eg paying bills by direct debit, i bulk bought laundry detergent, rice, pasta so i didn't have to worry about it for a few months. i stocked up on generic greetings cards and stockpiled a few gifts in case i needed an emergency gift. i bought my children's clothes a season in advance, when things were on sale, to save money, but also to keep me organised.

one of my Dcs was a bed wetter as well, so in addition to seeking medical advice, i bought a sheet to go on top their bed, so that i didn't have to to wash their whole bedding. i'd check their bed in the morning and throw the sheet in the wash before work, ready for drying when i got back home. in terms of friendships, could you nurture the ones your children already have eg play dates, or try and help them make new ones.

also, make time for yourself and your partner. when the children were asleep, i'd spend time soaking in a nice bath, watching a film with OH, reading or having a bit of pampering. You and your partner should give each other occasions time away from the children e.g an afternoon of free time once a month.

things will get easier, just keep telling yourself that.