I am so sorry u are goin thru this with your kitty. I have a 6 yr old male cat and he has stopped using the litter box for the simplest reasons. One he pooped on the floor cause I had turned the opening in his litter box cover in the other direction. Once I put it back he was fine. The last time, about a week or two ago he peed on the floor right in front of me! I couldn't believe it! He was about six inches away from the litter box too. I went and looked at the box and the bath bench I had over it, wasn't "quite" touching the wall...so he didn't want to go into the box. The bench is over the box and he would have to duck his head another inch to get in it. He still had lots of room but it was different and he did have 3 other boxes he could of used. So I fixed it and he was fine afterwards. My point, all though it be a long winded one,
cats can be very fussy if things have been changed one inch.