I've heard that Chika Umino has been going through some rough times herself. Get better Chika sensei, you don't need to strain yourself like that for the sake of others. You are doing your best.
@jimmysuzuki Ch. 161 and "the dark pit of concentration" I took to mean that concentrating on Shogi has been his escape from life, and he only got to be as good as he did because he wanted to escape all the time. But now that he doesn't want to escape life, he can't concentrate as well as he used to, both in preparation for matches and in matches themselves. Effectively he's powered by darkness, but he has less and less of that in his life to fuel his method of shogi.
I think somewhere soon (and what Hina is leading him toward, whether she realizes it or not) Rei is going make peace with Shogi and himself and play it for positive reasons rather than to escape negative ones. He needs to find happiness in Shogi rather than it being a survival tactic.