Oh hunny that was the most heart wrenching thing I have ever read. It is not your fault and nothing you did or didn't do would have changed this.
I don't know what to say except talk to your partner about how you are and how he is, tell him you are scared of losing him to. Be a rock for each other because your relationship is precious.
Most of all be kind to yourself, rest grieve, scream...do whatever you need to to get through this awful time. Thinking of you xo