haddock I have 3 close in age and they all had reflux disease.
A more serious form of reflux where they need Omeprazole in the first year of life to stop the stomach acid constantly coming up.
They screamed and screamed and screamed and fecking screamed.
I thought id go mad. It was one thing having it with a first baby but when I found out 2nd DC had it I just about died. She was the worst too. Never stopped feeding and screaming.
I had desperately wanted a girl after my boy but then I wanted to send her back. I kept saying things like "has she been sent to me in a joke" 
.
I thought I would NEVER get through it but I did. You Will, I promise.
I then found out I was PG with #3 
. Im sure I had depression for the whole PG and I was sure Id never cope.
Sure enough he had bloody reflux too. I was numb. I managed him too. Im still sane. He is 1 now and I actually enjoy them and all the guilt has all but vanished!!
When you see your two boys playing and interacting next year your heart will melt and you will have actually forgotten just how god damn awful it was. Reading your post brought it back to me.
I have some tips for screaming babies though as im an expert!!
Lie him on his stomach (sod the rules he won't die) in the moses basket with the head end propped up a bit with books. It is more soothing on their tummy and they don't get startle reflex. Make sure he can lift and turn his head - he will.
If he sleeps with you let him lie on your tummy on his tummy but make sure you have no covers or pillows near him and lie in the middle of the bed. I used to wear my fleece zipped open! (just in the early weeks though, they get too big too quick.
95% of cot deaths are in houses with smokers so if you smoke don't do the above.
White noise will end crying instantly. We used the hairdryer 
! This is an obvious fire hazard so don't fall asleep. We eventually got it downloaded. INSTANT!!
Don't overfeed him. They need 2 hours at least from the start of one feed to the start of the next. It is the only way his immature digestion system can cope. I've had enough chats with consultants about reflux disease to know this is the case and have experienced this with 3 babies so bugger off any lurking breast feeding natzi's 
. Joking of course.
Im not saying he has reflux, he probably doesn't but crying babies, ill or not, don't go on crying. It calms down and you will calm down and so will your 19 month old and he won't remember a thing about it.
I know you are thinking you won't make it to Monday like this but you just will. Then the following week will come and go and so on then you will suddenly look at your 12 week old lying in his bouncy chair smiling at his big brother and you will be having a coffee (a whole one) and it will suddenly dawn on you "oh, he has been a lot better actually).
Sending you big hugs. You will lift your head and breathe again soon enough. x