Lawsonia inermis Grows Gnarled Leaves

I ordered a few henna from Richters a few years ago and now I have one that I've enjoyed for over 5 years now, but in the past few months is has been very sick. The leaves are not developing right. They are growing quite quickly as usual, but they are gnarled and only live for a short period of time, then dry up and die. I have treated it for pests, pampered it as best I know, and even removed it from it's pot and rinsed away all the old soil and repoted it with fresh new soil just in case it was something in there. Nothing has helped. I have attached a couple pics of the leaves close up so that you can see what I'm talking about. I hope you can help me, I'd be so upset if I lost my henna now after all these years. I've been so looking forward to seeing it bloom someday.

The worst case scenario is that your plant caught a virus in which case there is nothing you can do, but burn the plant so no other plant catches the illness. Viruses are generally quite host specific, so the only way your plant could have become infected is if there was a related infected plant nearby and some insect or your ministrations transferred some sap from the infected plant to the healthy one. Not all plants show the same or even any symptoms when infected. Therefore just because all your other plants seem healthy is no guarantee that they are!
However there are nutrient deficiencies that have the same symptoms and you should eliminate that possibility before you take any drastic action. Get a complete fertilizer, one that also has micronutrients in it (hydroponic fertilizers are a good choice) and try it out for a month or so. Only burn the plant when the new growth produced with the complete fertilizer still shows the crippling and short life.
I have also seen an infestation of false spidermites produce crippled short-lived foliage. These mites are one tenth the size of ordinary spider mites and are virtually impossible to see with the naked eye. Spray with Kelthane or Pentac as they are unaffected by most other miticides. Be very careful with Kelthane because it is far too toxic to use without extensive precautions.