Something similar happened with me. Burnout? Too much anxiety about scoring high enough leading to decreased performance? That's what I guessed when it happened to me. Have you read the thread about burnout being real?
It sounds like you should keep reading The Economist; you're getting something positive out of it!
You might be a nerd, but in a good way. Now, if you have no reason to read The Economist other than for sheer pleasure, then you might be getting in…
I cancelled in December because my practice test scores were not high enough, and am taking it in June. I'm feeling pretty good about it, but I wish I had stumbled across 7Sage earlier than late April!