I truly believe difficult babies turn into angel toddlers / children
DS cried constantly as a baby. He cried in the car, pram, pushchair, bath, bed etc. My Dad used to say that it was like DS was protesting about simply having been born. He came out wailing and I remember desperately trying to give him his first feed and he was red-faced, screaming and he stayed that way for quite a long time. The only thing which worked was putting him in the sling and so I did. He lived in there for the first two years of his life.
He did get significantly easier as he got older but he never turned into the laid-back angel people claim he should be. He was a difficult toddler, easily upset and had the most terrible separation anxiety well into primary school.
He's 14 now, sleeps well and is funny, erudite, kind and good company. But he has always been sensitive, highly strung, prone to anxiety and prefers quiet and to be close to home.
Like I said, things got incrementally easier as time went on, particularly once he started sleeping properly but the promised miracle of parenting somehow becoming blessed and easy never quite arrived. Of course the myriad joys make up for it but I did feel a bit like I was chasing a lie for a time. Every time somebody said "oh he'll be an easy toddler/child/teen", I clung to that but ultimately, realising he was just a sensitive boy, prone to anxiety was what helped in the end because I could alter what I was doing rather than waiting for him to change.
Double check with the GP for underlying causes, have faith that it will get easier. Because it will, regardless of the why. Do whatever helps for now, call in all and any help and sleep as much as you can, as often as you can.