Need a sleep specialist

My 6 year old daughter won't sleep in her bed. She's always been an awful sleeper which led us to [unwanted] co-sleeping. By the time she was 4 co-sleeping wasn't working for us anymore and our marriage needed serious work. Over the past two years we have been struggling to transition our daughter to sleeping in her room. She will make great progress and sleep relatively well for 2-3 weeks and then inevitably bounces back to her usual refusal to sleep on her own, in spite of all our efforts to reward her good nighttime behavior. She wakes up and starts demanding that my husband or I sleep with her, won't take no for an answer or agree to be taken back to her bed. She will scream for hours demanding that we do what she wants. No amount of begging/negotiating/firmness gets us anywhere.  We have contacted our pediatrician but didn't get any suggestions that weren't obvious and already in use (such as reward chart, etc). We are absolutely exhausted and miserable. I wonder if anyone has been through the same and had any luck using a sleep specialist like Meg Zweiback, or someone else? I've read reviews and they are all written by parents of much younger children, little babies and young toddlers. No reviews from parents of school aged kids. These specialists are very expensive and it would be great to hear from someone who's had success with an "older" child, like my daughter.