The Doctor was familiar with guilt — intimately so, one might say, and they wouldn’t be wrong about that — and he’d found a way to live with it over and over again and knew he would again and again, but there was something sharper about this guilt, something deeper, something more.

The more he thought about it, the more he realized why this guilt hit him so hard in a way he hadn’t expected: it was because Clara was different and always had been — at least in this form, the one he’d subconsciously created for himself at his last regeneration — and always would be — in this form and every one after that, no matter how many dozens or hundreds he would be given in the end.

Clara was special because the Doctor loved her — which he had to remind himself, was the very reason for her downfall, the very reason she’d been there and been so brave and so self sacrificing — and because he loved her so much he would feel more keenly than anything before the guilt of what happened to her for the rest of his lives.