What is interesting in my opinion is the difference between his reaction to the death of Murphy verses Susan.
It is, that's one of the things that bothered me after Ghost Story, that we went to Harry staying on the island for 1+ year and then jump to Skin Game and we never see him deal with what happened with Susan.
I think what makes the difference (in part) are:
1. That the romantic relationship with Susan was long over, and it would have been anyway (by his own words) after she hid Maggie from him. His relationship with Murphy, however, at the time of her death, was his present, and also his future. He makes a point to say that he saw their future together die along with her. So, that would hit different. He had hopes with Murphy at the time her death happened.
2. Building on the previous point, the relationship with Susan was something that was the past, and it didn't work for one reason or the other. The relationship with Murphy was just starting out, it was full of expectations, he could still imagine/believe they would work. Loosing that is hard. (though, tbh, I felt Harry was pretty cold during what we saw in PT/BG about the relationship, like he was holding back a lot, but that's expected from a person with so much trauma around relationships).
3. He wasn't in love with Susan anymore. I dunno, he was more than ready to jump into bed with Murphy right after she died (which is kinda fucked up imo, but *shrug*).
4. Susan was in on what was about to happen, she (to some extent) consented to it in the moment, and it was a good cause, to save their child. Murphy, well, to be frank, she consented to the danger of it when she went out into the battlefield, but the moment itself is too sudden maybe?
5. As painful as it was, with Susan's death, Harry still won something, his daughter. He will always have Maggie thanks to Susan's sacrifice, and he has a part of her, even. He gained nothing from Murphy's death. You could argue that he's alive thanks to her, but in his mind that wouldn't count. And he has no children with Murphy, no piece of her left.
I think, the reason why Harry would grieve Murphy harder is because he was imagining a future, he had hope, he could believe that they had time to figure out and develop the relationship, but now her death robes them of that opportunity. With Susan, maybe he could file that under the "I already know it doesn't work", there's no hope there. Harry wasn't in love with Susan anymore.
Now, personally, I could add that Murphy is/was his greatest love, or at least, his more mature one, and that might play a part in his grief, too, in comparison to the death of Susan. I know it's canon that Harry and Susan shared true love, but if I'm judging from what I see, I don't buy it. I could buy the relationship with Murph as something more true.
Having said all that, I think Twelve Months is supposed to be about Harry doing the balance on all his loses, Jim said it was necessary, so we might see him dealing with Susan's death in this book too in other chapters.