Ali - it was like re-living my life when reading your post. I too had a 19 month old and a newborn.....and i remember the 6-8 week mark...there were lots of tears in this house !!!.
I guess the sleep cues are a biggie....we still wrap Sam and she is 4.5 months.
She isnt a big day sleeper.....i remember at 8 weeks some days she would have a 45 min sleep ALL DAY...i was beside myself.
I tried everthing...even putting the radio on whilst she was in the bassinet trying to get to sleep...pottering around the bedroom singing hoping she would drift off to sleep. Not much worked.
Like mickydan i would leave her, if she cried, i would pat, cuddle, walk out...it took a while but eventually she would go to sleep...
good luck....