What do you mean by, "I was getting tired of my nappy hair"
? Were you natural before the relaxer that you tried yourself or were you overdue for a touchup?
Anywho, in the six months that you've been transitioning, letting your relaxer grow out, have you trimmed or cut off the remaining relaxed ends? I'm no expert, but I believe six months should be enough time for you to grow out much of the relaxed hair in the back and any scab hairs. I think that unless you've been flat ironing or there may be any other damage (such as heat damage) your texture in the back might be natural by now, the way it is meant to be. I have at least 3 different types of curl patterns on my head and the back is much tighter, coily, and looks nothing like the hair on the majority of my head. It is not damaged, just different.
Perhaps you're the same? If the back is not damaged, cutting it all off won't make it grow back a different texture.
By, "the back of my hair is cutting rapidly short" do you mean its breaking or splitting? If you still have your old damaged relaxed ends that may have split, that split can travel all the way up the shaft and perhaps damage your new growth. If you haven't trimmed or cut off your old ends, maybe doing this could help.
I'm not recommending that you do anything in particular, as I'm not out to push you to make any decision that you'd later regret, but maybe starting fresh! You have six months of new growth, maybe getting rid of all the relaxed ends and starting a daily/weekly hair care routine may help to gain a healthy scalp and healthy hair.
If you WERE natural before trying to relaxer and what you wanted were looser curls as opposed to the natural texture you're describing, perhaps a texturizer may hair to achieve that. I've never had one myself so I have no tips but I will encourage you to do research, ask questions, and have it done by a professional
if you choose that path. Note that by having texturizer, you will be chemically changing your hair and you won't be 'natural' per se (I've learned there are different definitions of natural *shrugs*). I think you should start your research on these options here on this site. You will find a lot of ladies willing to help you out and links to other sites that may help, too. Post a photo if you can... I can try to help and I'm sure there are others who will, as well. If you have questions ASK!
Hope that helps... Welcome and Good luck!!