Sorry that you had a relapse after getting so much better. It's very frustrating.
I have had the same, I was recovering from a bad viral infection and overdid it and am back to almost step 1. I don't know why or how, I know in my case I overdid it and I also was eating foods that I was intolerant to (without realising it). I've been trying to relax and properly rest, eat well, take loads of vitamins. Sorry, I'm not able to offer more advice, but I think what we've both experienced is typical of recovery from something that's knocked the body for 6. We have to just let the body recover at it's pace, although it's very hard to do when used to just doing whatever you want. Hope that helps.