Just wanted t say, DD is 19 weeks, and sounds exactly like your DD - I was at the end of my tether, we also had a lot of waking at night...however, a week or so on, we are now in the position where she is soothing herself to sleep - if someone had told me that, even a week ago, I wouldn't have believed them. One day, she was just playing on her playmat, chuntered a bit, went into her and she was already asleep. In fact now, it seems if I do the rocking, or holding, it annoys her even more, she just likes to have a little moan, and then sucks her thumb until she sleeps.
I actually dont think anything we did made this happen, for what it's worth this is what we did do, consistently, from day one:
Dark and quiet at night, no nappy changes, no talking
music that she has when I feed her at night.
Bath and quiet feed upstairs, to let her wind down.
wouls get her ready for sleep as soon as she had been awake for about 1hr 30 mins - just by sitting quietly, or putting her in rocking chair with blanket over her
I havent tried the 'tricking' routine, but do pull the black out curtains and keep it quiet for naps if we are at home, especially did this when it was really bad, would feed her to sleep on our bed and then leave her to sleep there.
Good luck, but really, I kind of look back and think - did anything I did help? or would she just have got there on her own anyway?