OP - I am deeply sorry for your loss. You have asked for advice, so here goes:

The numbness is a coping mechanism and you need to push past it and talk to your GF. Even if it’s only once, she needs to see and hear your emotions surrounding this tragedy. Based on the information you have provided, here is how I would explain things:

GF, I am so, so sorry for the loss of our child. I know it’s easy to look back and think that I didn’t want our son, and maybe this was true in the first few days of your pregnancy. But I love you, and each day I was becoming more open to the idea of raising our child together. After he was born, I looked forward to spending time with our son, and even delighted in the fact that he looked happy to see me as well. I don’t understand why he was taken from us. I tried so hard to save him, and I want you to know that if I could sacrifice my life to bring our son back, I would do so without hesitation. I am so overwhelmed with grief by the thought of never seeing/touching our son again, and it’s causing me to say/do the wrong things. I am here to support you in any way that you need,