Sleep training is controversial but it worked miracles for us. I did the chair method, so for the first 7 nights I sat next to the cot and gave DD gentle rubs on the tummy every few minutes. Next 3 nights chair in the middle of the room, next 3 nights in the doorway, then after that not at all.
I never let her cry for more than 5 minutes without a check in, sometimes even less.
Nights 1 & 2 there was a bit of crying for 20 minutes, night 3 -10 minutes. By night 4 she had no interest in the tummy rubs and just snuggled down.
Moving her out of our room also was the best thing, because she sensed me and woke up.
She was 6 months, shes almost 3 now and sleeps like the dead. 7.30 - 6.30 and has to be wrangled out of bed in the mornings
The way it was explained to me was that when they use a dummy, boob, bottle or rocking to fall asleep they don't understand how to sleep without it. Moreso as they get older. Everyone wakes up multiple times in the night but we don't know about it because its for such a small amount of time that we go straight back to sleep. Little ones who need something to help them fall asleep cant do that, so learning to fall asleep without that aid is the key.