@cyantist
We did controlled crying. It's not a pleasant experience but it worked very, very quickly. DD it was about 3 nights of crying, 3 nights of whinging, then she started chatting herself to sleep. DS was similar, maybe even quicker. The change was immediate, however. They were so much happier and so were we!
This is what we did:
Put in cot, say goodnight, leave the room. If they're crying, go back in after 1min, reassure (without picking them up!), leave again. Then wait for 2mins, and so on. We agreed that 10mins was the limit, and if we got to that point we'd repeat 10mins, but it never happened - never made it past 7mins because they were always asleep by then. Some people do 2min increments, some people do 5. But we did 1min and it seemed to work.
We also took turns to do it, so they got used to DH putting them down as well. That was really important because prior to that it had to be me or they wouldn't fall asleep. Mine were both FF, which probably helped.
It doesn't work for everyone but it's worth a try. We were on our knees, didn't feel safe driving, etc. The DC were miserable because they weren't getting enough sleep, and nobody was enjoying life! CC changed all of that.
Best of luck 