well, you sound completely normal to me.
I was just like you, always stressed and shouting too much, and feeling totally and completely guilty about shouting so much.
Then two things happened.
My eldest ds told me I shouted too much, and that life would be great if I didn't shout so much ( he was about 9, dd was about 6. ds2 was baby).
So I listened and since then have tried not to shout. (v v hard!!)
I endlessly beat myself up about what a rubbish mother I am, but my kids laugh (21, 18.12)>
The thing is, they always love and forgive me, and it's important you know this too.
And tis v v v v v important that you find great, happy, fulfilling things in your own life that give you the perspective to know how great you are, and how great it is to have your kids, and to be happy together 
believe me, i speak from experience! x