Leesha I would look into a sleep school and see if that helps. My DD was a nightmare sleeper (your story is bringing back some bad memories) and we went to sleep school. She was so bad that we actually failed sleep school and had to keep going back for three days in a row (ours is only a day time school). They told me it should take 3 nights before you see any improvement. Then it gets better and better each night. My DD was so stubborn it took a week to see the improvement. Now she is an amazing sleeper - sleep school was the best thing we ever did, for her and us.
Consistency is the key. Set her settling routine and stick to it every single time you settle her. You and Shel have to do exactly the same thing so Jazz knows what is happening. DH and I would try something different everytime and we were each doing something different so poor DD didn't know what was happening. Set your routine and stick to it. It is so incredibly hard in those first few days but after a week you have a beautiful sleeper and you enjoy being a mummy again.
Best of luck gorgeous. Let us know how you're going.