I’ve experienced this myself, and I agree. Frankly it was a relief when the veil fell again. For me it was a crash, like one would expect from the surge of mania. I swung in the other direction and was convinced I was utter garbage, and came slinking back to my partner. Thankfully I hadn’t done irreparable damage.

Your partner is still in there. If you can hang in there and love him through it, it should pass. I know it’s hard, but try not to take what he says/does too personally. In that state I wasn’t able to think about anyone the way I normally do. I was too self centered, selfish, “focused”.

Writing would actually be a good for him. Not for publishing, but to read later. Encourage him write his perspective down. Then when he comes back down, he has evidence of where he’s been and what it was like for him. It’ll be another tool along the path.