Hi OP
My goodness poor you.
I know how it feels, that sick and heavy feeling when you start to come to terms with that kind of childhood.
Much of what you say has echoes for me, too. My parents' relationship was AWFUL, my mum had (I think) undiagnosed depression for much of my childhood and this manifested itself in screaming rages. Their rows were terrible, my dad would yell at her that she was mad and needed to see a psychiatrist but he never actually DID anything about her temper or her misery.
Like your mum, she could be lovely but she was mostly either in a very irritable mood or actively aggressive. We would get screamed at, for up to half an hour on end, if we did something that upset her - the sorts of things that upset her were not getting the right grade in music exams (she once screamed at me that I probably hadn't done better because I was 'so ugly' that the examiner hadn't wanted to give me a higher mark) and not coming top in schooll tests.
We would also upset her to the point of screaming violence if we expressed any bad moods of our own, she once screamed at us that she was going to drive us all into the back of another car and kill us all, because there had been a bit of argy-bargy getting out the door for school one morning (it usually ran liek clockwork). WE were aged 6 ish to 10ish.
My dad did nothing when she went into these rages with us and, as I say, when they had their own massive screaming rows, he would just yell that she was mad and needed help 
She would hit us too when we were quite a bit younger, it stopped when I was about 5 but by then we were well trained enough not to do too much to upset her.
All our parents rows would end in my mum storming off somewhere and us all being in tears trying to bring her back. Often on holiday, like your situation.
Sometimes when they had rows late at night when they thought we were asleep my mum would throw things and break things. And slap him. Once (I think) he hit her back.
We were very very middle class, they had profesisonal jobs, and nobody would ever have thought this was our life.
More to the point, I never knew there was anything QUITE so very wrong or abnormal about it until I was an adult. I mean, I knew it was a stressful, walking-on-eggshells childhood but I couldlnt' have verbalised that.
I had an eating disorder when I was in my late teens/early 20s, and pretty much a complete nervous breakdown in my early-mid 20s.
I think the sick, heavy feeling you describe just overwhelmed me by then.
I had a lot of very good therapy but some of that made the sick/heavy feeling worse. I remember after one session, where I talked for the first time about my childhood, feeling like I had been punched and kicked, I felt physically bruised by talking about it.
I'm so sorry you're feeling this way, I don't know if you have chidlren of your own but I thinkk that can be a time when your own childhood comes to the fore.
I am better now, I have come through it, but coming to terms with it was difficult and painful.
However, coming to terms with it has made me a much better mother than I ever thought I'd be capable of!! I was very anti motherhood as I thought it was just a grim thing and that you automatically ended up with children who were scared of you/felt beholden to you... little did I know!!! Now that I have DC of my own, I know that this is not the case.
People who say that your parents did the best they could are trying to help, i think, but I don't actually think that is helpful to you. I think maybe you need ot be allowed to feel furious with them for the way they treated you. Thinking that they 'did their best' doesn't always help you to be furious.
In my case, my parents didn't do their best, frankly. Yes, they both had emotional problems. But that was NO excuse or explanation. They always put on a very different front for friends/family. We were the perfect family, in otehrs' eyes. They knew they were making big mistakes, my dad clearly knew my mum was unstable but he did nothing to help her or protect us. THat isn't him doing his best.
Sorry this has been epic!!! Do PM me if you want!
Just wishing you the best, really, and saying that I understand. It sucks, big time, but once you are through this feeling, you WILL be able to move on and be stronger for it.
x