I could have written this post myself, except add 16 weeks pregnant plus a 4.5 year old to the mix. My nearly 18mth old is doing the same except he goes to bed and to sleep fine, but wakes between 1am and 3am screaming this could last an hour until I bring him to bed with or quiet often he has carried on for over 5hrs. Now he even mucks around and carries on in my bed, so he ends up in the cot screaming. I too am so exhausted. No advise, I've tried camping in his room, just laying him down, patting, music, even tv. I even just ignored him still he was to stubborn to just lay down to sleep.
last night he was a bit better, he started at 4am, bought him into bed with me, he mucked around so at 5 his was but back in his cot to scream 30mins later there was silence.