OK sorry! So my ds2 is now almost ten months old and is a delight but I'm sorry t0 say it took until just a few weeks ago to get there.
My first (who is only 19 Month older than ds2) was a delight so ds2 was a shock!
He screamed all day for months. I tried everything. the only consolation was that he was a decent sleeper at night.
We went back and forth to doctors, hv, paediatricians at the hospital and had it all told to us, colic, then teething then just a grumpy baby.
about a month ago his urine was tested and he was given antibiotics for an e coli uti. he has been fab ever since. I don't know if that was an issue that had been going for a long time or if it was just a turning point but it might be worth to have it checked.
I don't have much advice I'm afraid, just a massive hug and tons of sympathy. it is unbelievably hard dealing with a baby like that. But a few things I would say..
don't lie to people that ask how you are doing. i got so much more help when I started to say how hard it all was.
take as many breaks away from baby as you can get
keep cuddling and treating baby as you would if they were happy - its hard to explain what i mean there as of course you love and care but it really helps to feel normal - I hope you get what I mean?
I switched to formula at 6 months - it was best decision for both of us and i refuse to feel guilty about it!
don't worry about walking away from them when they cry. it's too much sometimes. I still actually cannot bear it when ds2 cries the sound brings back all those awful early days.
ds2 did have phases over those months when he was 'normal' - I took full advantage and sucked up every laugh and smile.
I hope you don't have such a long slog ahead of you but just know it does get better