Well, re: first reply, it makes sense to me. He sounds as if he placed a massive amount of pressure on himself and had extremely high expectations of his father role. Perhaps it was due to his experience of his own father/lack of resolution, as you say. Interestingly, my BF also had an absent/unknown (as in never met) father and was raised by a step dad. When the crisis thing happened to us recently (which is related to children) it also seems to have unleashed a crazy/unexpected deep reaction relating to unresolved issues with the absent father. This has then triggered a breakdown.
I can totally see how your H's experience of the birth and the NICU, coupled with trying to support you, would have caused a lot of trauma and emotions that he has potentially never dealt with. It is very difficult because even if you acknowledged them and tried to help him talk about them, sometimes people don't want to 'go there'. They have to want to and we can't do anything about how they deal with it other than observe/be supportive/listen etc. It is hard to adopt such a passive role, if you think about how damaging their not dealing with it is.
I believe you should be kind to yourself as well though. You said:
I felt, and feel, like such an enormous failure that he spent every moment making sure I got all of the "firsts" to try to make up for what I felt I had lost. He wanted to try to make it all better for me, but in doing so he lost out on stuff for him. He spent ages trying to get her to say "Mama" first (she did).
The thing with such times of stress, trauma and unknown/unexpected things happening is that we don't know how to react. Everyone seems to deal with it differently. It's like trying to navigate an unknown route without a map! No one knows where they are going. You may have taken on the passenger role and he may have taken on the role of driver. Your reaction doesn't make you a failure though. As far as I can see, you both played a different role in dealing with what was happening. He chose to play that role, which was very lovely of him, but you are not the responsible for him having chosen to do that. Just as he was not responsible, really, for ensuring you got those firsts. He chose to do it presumably because he wanted to. So as far as I can see, there's really no right or wrong to have failed at, is there? Maybe you feel like that now because you are attaching an outcome (as you see it now) to your role and behaviour then?
I am inclined to think that OW presented an escapist route, as in being with her is escapism. My BF has also chosen an escapist route, although not with an OW, he has essentially left me behind whilst he tries to sort himself out. He has kind of run away. I expect this is not unusual. I think both you and I have felt we'd like to run away too at times!