Hi OP – I’ve been through similar and totally understand how you feel. DP has a son from a previous relationship, I have no children. It’s never been massively high on my agenda, but I hadn’t ruled it out – DP knew this when he met me and we have gone through phases of ‘wouldn’t it be nice’ and phases of ‘no way’. I’m in my late 30s so time is quickly running out for me. I fell pregnant accidentally last year, and it was a huge shock – I had pretty much convinced myself I was infertile for a number of reasons so it hadn’t even entered my head that this could happen. It was totally and utterly the wrong time for us because of various things we had going on in our lives. I also have a hereditary disease in my family that meant any pregnancy that hadn’t been planned in advance and monitored by a doctor could have ended badly (don’t want to give too much info here in case I out myself). So, I terminated. I was, even on the day, and with all the reasons I stated, uncertain I was doing the right thing. DP was certain we were, and every time I wanted to discuss ‘what if’ in the run up to the termination he reminded me of all the reasons why not. A year down the line, I am not really scarred by the decision, or by the termination itself, with hindsight I do believe it was the right thing as I am still not convinced now that I want children or that I would have been happy with a baby. But I do think a lot about where we would be now if DP had even uttered one word of encouragement or given any indication that he would be happy to be a dad again, instead of being horrified that it had happened and adamant that we had to terminate. I think the result might have been different. Not saying it would have been the right result, just different.
His son was not planned; his exW insisted on going through with the pregnancy even though he wasn’t keen as their relationship was on the rocks, so he had been through this before, and I am pretty sure if he hadn’t he might have acted differently when it happened to us. I also think having already been a parent with all its ups and downs and stresses (and he doesn’t cope well with stress) influenced whether he wanted to do it again – if he hadn’t already had a child I suspect his reaction would have been quite different. He says he loves me way more than he ever loved her, and I do believe him, but a small part of me still thinks ‘well you had a baby with her, why not with me’, even though I know that’s ridiculous, having a baby with me wouldn’t have meant that he loved me any more or any less.
And, EOW I have to watch him being a dad to the first child that was unplanned, whereas ‘my’ unplanned child never got a chance. It’s tough, really tough. And when he says he misses SS, or is upset about something to do with him, I feel kind of numb, a total lack of empathy, which wasn’t there before. If I ever get upset about ‘our’ baby I get a hug and a ‘but you know we made the right decision’, and the conversation is over. I really don’t think he ever even thinks about the ‘what ifs’ like I do, it’s over and done for him. For me, it has changed our relationship, and I am still trying to work out what the end result is, for him I suspect nothing has changed. We don’t talk about it.
Sorry no words of wisdom, just empathy. It hurts. But for what it’s worth, after 3 months, I was still a hormonal mess, crying at the drop of a hat, getting upset over silly things. That has changed, and hopefully once your body settles back down to normal the raw grief may fade a little. Mine has. I still have the 'what ifs' and the resentment, but it is bearable now. 
for you.