As someone who works full time including overseas travel with three DCs and a school and PTFA who get up to a lot, I agree it is a fair bit of work but think that it doesn't need to overwhelm you like this.
You say you are organised but also that you 'dont remember half the things'. That's key number one - none of us do, so like pps have said, you need a calendar that is the family bible- some of my friends use Google, some use the Organised Mum one, I have a cheap one from Tiger. Enter the termly dates, activities and any clothing required for school, work travel dates, anniversary dates, your haircut dates, EVERYTHING in the calendar as soon as possible, check book bags every night and rsvp for parties, photo invites and add to immediately etc. Keep it up to date if you know on a Monday that you'll need to work late on a Wednesday etc. It sounds like a lot, but once you've got it set up it only takes a few minutes to update and check and saves so much pain.
Get help with the house if you can afford it.
Clubs - it also seems like a lot of your stress is coming from outside activities. Is only your eldest at school? It seems like a lot, as are two scheduled activities on the weekend. Ours get one sporty and one 'culture' before / after school activity each term, and we have sometimes added one on the weekend like swimming lessons. Beyond this it can be too much, let alone volunteering. Are there other ways you can contribute and feel better, like financially or by helping with a fundraiser? Kids need unscheduled time too and if you are able to get sorted and maybe put less pressure on yourself to do so much you will all be able to relax a bit more.