Was it like that since you started writing and going to school or did it start recently? Maybe something happened and you're stressed aboutv it.
Or maybe it's the beginning of dislexia.
Just think about it, I hope you don't think backwards either.
I'm homeschooled, but, uh, the backwards writing happened ever since I started writing, but then I was doing really good, and now this. Maybe it is stress, I have a lot of things on my mind, now.