Hello
Sounds like a really stressful day. Glad you were feeling a little better.
It sounds like you have done a really good job of building up a business with 2 very young DC, so well done on that.
I have been on my own for nearly 4 years now.
I find there are peaks and troughs. I am not sure school has made much difference to me, as DS was in nursery and now he is in school he goes to wraparound care.
The advantages of running your own thing is that you can fit into 'school life' a little more easily. I have found that a lot of meetings happen during school hours and of course xmas plays, sports day etc are all during the school day too. It is also helpful to 'get the school gates' a couple of times a week to help to know the other parents (esp those of your DC's friends), the teacher & to see the notices that the school puts in the classroom window which is very helpful for parents that work full time 
I would echo what hazel says. If you are busy it could be worth thinking about what you can outsource. If you don't want to increase your hours, then maybe try ensure that the hours that you do have are being used in the best way. I have used Virtual Assistants (VA's) before and for c£12 - 15 per hour they can do all sorts for you. I have also used www.peopleperhour.com to find people to do things for me.
I have also started to use a business coach recently which provides me with an outlet and helps me feel like I am moving forward etc. I have found it really helpful.
I also have a 'buddy' which I met on this board and we catch up every few weeks via phone we are in the same sort of area which helps.
I would try to develop a network if you don't have one. A lot of my friends don't really get it either, DH is very supportive and interested though which is helpful.
When I feel like you did yesterday I basically cling onto the idea that the flex is worth the stress.
I find it difficult to take it easy when I don't have as much on, but I really force myself as there are days when I cannot take it easy.
Finally, I find it all an ongoing 'learning journey' and if something doesn't work (eg calling HMRC with the DC about) then I try to remember not to do it next time. I am not sure you can be taught any of these things, we just have to work out what works for us etc.
HTH & good luck.