This post will seem 'all about me' but I am hoping you recognise something of yourself, that I can think through strategies and they might be a good idea for you too. Also that you are not alone.
I got that leaflet, am EXACTLY the same, similar age gap between my two. I signed up interested in a parenting course but need to follow it up. I am a lone parent and work part time, I get some time for myself once a week when children are in nursery and when I go away overnight for work every few weeks (I pay for this 'time off' with more killer tantrums), my mum and sister babysit everynow and then, and it makes me feel guilty that they do.
I hate to accept help of any type. It makes me feel like a charity case who can't cope. Now that is silly.
Is your shouting at one child in particular? With me it is my eldest (3). He is what nice people call 'spirited' and I call a massive PITA. He is very, very hard. I guess what I am getting at is, I often feel frustrated like his behaviour is a result of my shoddy parenting. But DS2 is nothing like it, he is an easy child to love (18mths, at this age DS1 'turned naughty'). I, and perhaps you, need to think about our expectations in terms of behaviour, rather than get into the frustrated with self, anger at child cycle.
I particularly need to avoid 'flashpoints' don't let him get even a little hungry, don't take him out when tired, recognise a grumpy day, don't let him hold his wee. If I break those rules, I suppose I need to prepare myself for the consequences rather than becoming infuriated.
I learnt to recognise when I was getting wound up. Shallow, fast, breathing; blood rushing to me head, white noise in my ears, tense muscles everywhere.
I think on balance I am better than I was. But DS had a massive melt down trying to get him out of an unwell friend's house today. I could have wept. I am so EMBARRASSED by him. I just had to lift him it without coat or shoes and deposit him screaming into the car, whilst being silently watched on in horror by SuperMumWithPerfectLifeHomeHusband4DelightfulChildrenAndAnEducation. He had already stuck the place out by pooing his pants, weed in defiance. I shouted at him in the car, not full volume, held self back "I actively dislike you at times" whilst hissing "I could bloody kill you" (nb just an expression!) under my breath.
I just think other people are judging me all the time. They probably are. Other times the shouting is more likely to happen are before my period, like you when under time pressure, when having financial mishap (eg declined card unexpectedly, tax credits over payment etc). Identify and prepare would be a good idea for me.
I laugh at the leave the room idea. Yeah good one. They either follow me (still screaming) or start on each other. I have screamed into a pillow on occasion. I literally had to run away. And yes, the shouting feels like a discharge of the stress of the situation, but I, like you must find less damaging alternatives.
That said, I think children of yesteryear were treated far harsher and generation on generation educated parenting improves. Also, you may not be shouting as much as you think. I have been convinced I was shouting constantly at a friend's house, whereupon they said "you are good, you are firm but don't shout" I thought they were being sarcastic, but no. So may not be as bad as you think. I know I do shout horribly at times. It's my alternative to hitting, but probably just as bad. I hope it gets easier for you. Truly, PM me if your want to commiserate with someone!