Hi Aradialv,
Welcome to the Mattress Forum! 
I’m sorry to hear about your back issues. 
I know how demanding it is to be a professional dancer and the toll it takes on your body.
Unfortunately, there is no one “best” mattress or material for someone with a bad back (regardless of some of the things you may have read online). I wish there was! In general … what you would be looking for is a mattress with firm enough deep support and then comfort layers that are “just enough” in terms of thickness and softness to “fill in” and support the more recessed parts of your sleeping profile and relieve pressure in your most pressure prone sleeping position (usually side sleeping for those who sleep in this position) so that there is less risk to alignment in your other sleeping positions. You can read about the two main functions of a mattress here.
Two of the more important links in the mattress shopping tutorial that I would especially make sure you’ve read are post #2 here which has more about the different ways to buy a suitable mattress that is the best “match” in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) that can help you assess and minimize the risks of making a choice that doesn’t turn out as well as you hoped for that are involved in each of them and post #13 here which has more about the most important parts of the “value” of a mattress purchase which can help you make more meaningful comparisons between mattresses in terms of suitability, durability, and value.
While it’s not possible to “diagnose” mattress comfort issues on a forum for a “bad back” with any certainty because they can be very complex and there are too many unique unknowns, variables, and complexities involved that can affect how each person sleeps on a mattress in terms of “comfort”, firmness, and PPP or any “symptoms” they experience … there is more about the most common symptoms that people may experience when they sleep on a mattress and the most likely (although not the only) reasons for them in post #2 here that you may find informative.
Because of your unique circumstances and your petite frame, it will be important to find something with adequate “deep support” to help you maintain a more neutral alignment (this could be from innerspring, polyfoam or a latex support core), and then find a plush enough amount of comfort foams that are durable and hold shape well. Being petite you will probably need slightly softer materials, but you’ll want to avoid too much of this material, as you don’t want to “sink in” too deeply and negatively impact your alignment. You’ll also want to make sure that the pillow you’re using is appropriate to help maintain a more neutral alignment with your cervical region.
Because of your specific concerns, you may be best served by looking and testing out items locally. If you choose to go that route, make sure that you find out the information listed here so you can compare the quality of the materials and components to the durability guidelines here to make sure there are no lower quality materials or weak links in a mattress that would be a cause for concern relative to the durability and useful life of a mattress before making any purchase.
If you have any more specific questions after reading through this, I’ll do my best to be assistive.
Phoenix