Lists.
I have lots, different ones for different things, but that's just me. For anyone drowning in the minutiae of life, I recommend two.
#1 is a master list. Get an A4 notebook/pukka pad/whatever, or do it electronically. Write down everything you need to do. Everything. Make dentist's appointment, costume for superheroes day, repeat prescriptions, research new jobs, clean fridge, call mum, email school, call CAB, do laundry, pay phone bill, sort kids clothes for outgrown stuff, move house, sue builders, write condolence letter, etc etc. Subcategorise the line items if you must for the sake of getting things straight in your head, but it's not necessary.
#2 is a day list. Each evening, in conjunction with your diary, make a list of things you'll do the next day. This might be because there's a deadline, or they're about to go toxic and blow up in your face, or because they've been on the master list so long they make you feel like a loser. Or they might be pragmatic choices, e.g. you're meeting someone round the corner from the pharmacy and can dispense prescriptions at the same time, or you're working from home in the morning and will have the P&Q to make phonecalls and just enough time to get a hot wash done and on to the line.
#2 list can go with you everywhere in your back pocket - no need for a bag, OP. It can also act as anything from a do-or-die list, to a menu you can pick and choose from depending on how the time goes - your choice.
This is the important bit: forget everything that hasn't made it on to #2 list. This will stop you feeling so overwhelmed you don't know where to begin.
Every night, cross off what you've managed to get done. #1 list should get smaller, although obviously it will also have new entries on to it. Once a week, rewrite it completely. Anything that keeps getting rewritten week after week needs looking at to see why it's not getting done. Maybe you need to delegate it or get help with it, maybe it needs breaking down into constituent tasks, maybe you don't really want to do it at all.
I've been organising myself like this since last autumn, when I decided to start my own business on a shoestring because of an opportunity that came up. It meant working full-time as well as having 3 school-aged kids and doing the bulk of the household-wrangling (because DP works more-than-full-time already). So far so good, although the house is still a mess, and the kids still say things like, 'I should probably have told you this earlier...' 
Hope it helps someone, anyway.
Why is this thread in Relationships??