Dd was much the same and very little worked for her. We tried infant gaviscon in the milk, early weaning (which didn't work and we abandoned), carrying her around, pushing her in a pushchair... nothing seemed to soothe her. It was awful; there were times I really, really felt like chucking her out the window and I had to put her down and leave the room
. The only saving grace was that she slept a solid 12 hrs a night from 4 weeks. Those evenings and the good sleep kept me sane.
I'd have loved to have been able to sit and nurse her all day but, with a toddler and a house to run it just wasn't practical.
In the end she improved around 6months when she was ready for solid food. Her reflux improved, she got into a routine and she stopped throwing up everywhere. She became better still when she learned to crawl and move around by herself so that she wasn't crying in frustration at not being able to get anywhere.
I also felt I'd forgotten ds and that he was more or less completely ignored as I was struggling to keep dd on an even keel. I felt so guilty, like I'd made a massive mistake and ruined all our lives.
I'm not sure if these suggestions would be of any use to you but these things helped dd settle for longer than 5 mins at a time and have scream free windows during the day:
Swing chair
Bumbo seat
Swizzley seat thing with toys around
Door bouncer
Going out for a drive or a long walk in the buggy
Basically anything that kept her upright. As soon as she laid down, she cried (apart from bedtime, weirdly).
It might not seem like it now but it will get better, there will come a time when your dd's happy side will come out. In the meantime do whatever you can to get through. Give your ds the time he needs with you when your Dh/p is at home, if possible, and try to focus just on him for perhaps ten mins. It is heartbreaking to hear them cry but your dd won't remember if you leave her to cry whilst you attend to your other child: it's important to make him feel he matters too.
Sorry if it sounds all garbled and senseless, I'm just trying to get across that you're not alone and that this stage will pass.