I am not very flexible either and completly understand all of your frustrations.
I tend to be more flexible in my hips than my back and always assumed that flexibility is some magical innate talent that some people have and some don't. Although some people are more natually limber, we are not without hope.
My little discovery started when I was working with my instructor on "winging" my foot for coupes, arabesque, and attitudes. When she tried to shape my foot I would always say, "I am just not that flexible" as I could barely do a wraped coup de pied at the time.
One day she pulled me aside and had me sit with my legs straight in front of me, toes pointed. She had me wing my foot as much as I could. Then she said look at your leg around the outside of the knee. What I saw was my muscles moving back and forth winging my foot. She said although winging the foot may come easier fot someone more flexible, you can strengthen your muscles to achieve the same thing. It maybe a lot more effort in the begining, but as your muscles gets stronger and the muscle-memory increase, you will be able to wing your foot every time.
This little tidbit changed my whole preception of flexibility. Rather thinking of flexibility as being able to be the human rubberband, think of it as strengthening your muscles with repeated movements enagaging those muscles every time.
I started doing a 20-25 minute floor barre before every ballet class. And after about 6 months of 4 to 5 times a week, I can say that I noticed a remarkable difference in my hip alignment, flexibility in particular to my back and can do a split , although not very comfortibly. I don't think I am actually more flexible but rather my lower back, inner thigh muscles, and glutes are just stronger to hold what needs to be held.
I hope that was worth your time reading and was some what helpful.
Good luck and keep trying!