Hello all I really hope someone can help & apologise in advance for the essay 
I have 3 DDs (aged 13,8 + 22m) and am 6 months pregnant with DC4. Am at the end of my tether with DD3 not sleeping to the point I'm actually crying as I type 
A litte more info...DD1 slept poorly until 18m but great after. DD2 is Autistic but has slept ok since around 7yo. DD3 is nearly 2 but DOES NOT SLEEP THROUGH YET!!
Last night I was up 8 TIMES with her! Being pregnant with the 4th baby I am exhausted and have been forced to take early maternity leave as I was struggling to cope day to day
DH is an HGV driver so is away for days & works funny hours.
DD3 has had poor health since she was born with several hospital admissions thanks to a compromised immune system so I'm certain the ups and downs have contributed. In herself she is incredibly content and very advanced, developing wonderfully, speaking in sentences etc so I have no concerns there. She is an absolute joy. BUT she wakes several times nightly, every night. She has a natural diet & I have altered her whole routine to try to remedy the issue as follows:
Limited or no sleep during the day (max 45 min sleep) as the longer she sleeps the worse it is!
Bathtime early eve with lights dimmed & cuddles to wind her down with a story & warm milk
Bedtime between 8-9 pm where she always falls asleep straight away! She only gets her bedtime bear (had since birth) when she is actually in bed so she associates it with sleep time
When she wakes through the night and stands at her gate crying, go in, put back into bed (no speaking) cover her up, kiss & back out. The only words we ever use are "sleepy time now baby".
We NEVER bring her into bed as tempting as it is! She has been in a toddler bed for about 5 months as we thought she may be waking due to banging the cot sides as shes very tall! She LOVES her pink flower bed & her room!
I relented on Sat night and begged MIL to have her overnight so I could sleep but it just made things worse as I was told "well she slept for ME!!"...but thats quite possibly because MIL had her in her bed after I said NOT TO!!!! 
I feel utterly silly as a mum of 3 nearly 4 that I can't even sort out my DDs sleep but I am on the edge now...and the thought of having a new baby in 3 months who will be breastfeeding through the night AND dealing with DD3...well I could cry! And bearing in mind I'll have to be up & out the door to take older DDs to school with the babies by 8am...arrggghhhhh!
As sorry as I am to rant on...I feel relieved that I have got it off my chest. SO grateful for mumsnet! 