Do you have a partner? If so and they're not doing 50:50 of everything (childcare, chores, life admin etc) they need to be. This includes splitting the mental load - if you have to tell DP what needs doing, that doesn't count. I find if you're working as a team, everything starts to feel better.
Find as much childcare as you can access/afford. Anything - grandparents, play dates in friends gardens, pay for time at nursery/childminders (anything, even one morning a week helps), gym with a crèche, babysitter.
Schedule an activity every morning first thing. Ditch the chores and straight out the door to something where you can speak to other adults and/or get a coffee.
Get loads of little activities for them to do in the afternoons - cheap stuff like craft kits, playdoh, colouring. Stuff that doesn't take a lot of mental effort for you I.e open bag of craft, let the kids have at it, Chuck in the bin after.
Enforce with absolute rigidity getting up time and bedtime. I don't allow kids downstairs before 7am and they have to be quiet before then. Keep bedtime early.
Insist on doing things that you enjoy as well e.g. I enjoy reading to mine. It doesn't cost me any mental effort, so we do that regularly whether they like it or not.
Basically share the load, schedule the hell out of everything, rules, rules, rules and put yourself first occasionally.