Very sorry you had such a bad night and first experience. It is terrible to have a night like that. When I first relized I'd have to get all these stones out of my liver, I was scared to flush for just the reasons that you experienced. And, even more, I was terrified to get a stone stuck on the way out. Just the thought of drinking any oil (let alone 1/2 cup) made me nauseas. I was already extremely congested.
I've been approaching it this way, and it's worked very well for me. The idea with all of it, is to get congestion out of your system before you flush so the stones can pass easily. All that sickness you had was because of the congestion in your liver and probably your intestines too. It wouldn't allow stones out, or the oil to digest and move through. And so you were horribly ill.
Here's what I've found that works:
1. Do 5 or 6 days preparatory, drinking 1 litre of apple juice, and keeping a diet that won't bunge you up, ie, mostly fruit and veggies (no dairy, eggs, or meat). The flush by hulda doesn't stress much on preparatory diet, but Andreas Moritz does. Personally, I like his approach. It's another precaution for me for not getting ill.
2. the day of the flush, if you can have a colonic, that's best, but if not, a couple good enemas will do.
3. make sure to drink lots of water that day because Epsom Salts will dehyrate you.
4. take the doses of epsom, and only use 1/2 cup oil, even less may work. My first flush, I was scared, 1/2 cup still seemed like a lot for me. So, I tried a 1/4 cup. It still worked and I got out about 50 stones, some as big as blueberries.
Sometimes when I flush I don't get any stones out, but I'm never horribly ill. And believe me, I'm full of stones! Even if you don't do all of this, just do the best you can, and go easy on the oil.
It's a tricky thing learning how to heal yourself. I think many of us have suffered needlessly because we didn't know how to best approach our problem. I have too. The number of nights I had crouched on the floor in gallstone attacks was something I now know I could have avoided and remedied. I didn't have to suffer, but I didn't know then.
Hope this helps a bit, you're doing the right thing in healing yourself.