I also had a second child born 2 months early who had trouble sleeping. What I finally figured out (for my son) was that during the day it was loud in the house and at night the house was too quiet. I put a fan on to help create some noise. Anytime he slept, day or night, I used the fan. You could also try a sound machine or a cd. Anything that is calm and relaxing. Once I created noise at night he started sleeping better. It still took some time to get him to sleep through the night because it had become a habit.
I am sorry that so far nothing has worked to help your child sleep through the night. I bought a book 'Solve your Child's sleep problem' by Richard Ferber, M.D. I haven't read the entire book but the parts I read were very useful. This book explains in detail the different phases of sleep and importance of each one. Plus it talks about many different sleep problems (medical, schedules, associations, disturbances, etc) then it gives you some solutions to each situation or problem. I hope this helps. Good luck.