My dd did the 1.5hrly wakeups for 6 weeks, it started around that age. She settled down on her own, thinking it was just a phase. I can sympathise, it's not easy at all! We were co sleeping and found it was time for her to be in her own bed. She slept much better but was still waking alot, moved her to her own room and she now sleeps through. I think she could smell the milk or at least knew I was right next to her and that the milk was just there.