Sorry to read about your hair problems. From what I understand everyone's hair grows at a different rate. The hair on the back of your head does seem to be growing slower than the hair on other parts of your head and from what I've learned that happens as well. During my transitioning, my relaxed ends broke off while the new growth came in, and after a while when only bits of relaxed hair was left my mom began to trim those ends off. I didn't retain all of my length but most of it.
I say give the back of your hair time to grow in. I know its frustrating but eventually if the damaged hair doesn't break off trim it. You can also design your hair so that the back of it is well hidden. I suggest cornrowing the back and if protective styling isn't your thing you can use the rest of your hair to cover it up (depending on its length) to give it time. Hope this helps.