Oh, bless you. You poor thing. Please hang on in there. This will definitely get better, but right now get a hot water bottle and wallow in Netflix and cry. I'm sorry but you just have to get through this bit somehow.
My goodness. What a situation. So, if I'm understanding right - you and he have a 3 year old son, and live together with him. Your partner also has a 12 yr old daughter, and she's with you guys quite a lot of the time - 3/4 days a week. Now you're pregnant and he is totally against it.
OK: first thing. You absolutely want the baby and don't want an abortion. So, you need to stick to that. The hospital staff are quite right. They would hate to do this to you. And you have depression. OMG if you have an abortion as well, it will finish you off (I don't care what the pro-'choice' MNetters say to this - they'll crucify me - but it's what I think/know, so stuff them). I had an abortion against my wishes when I was 21 and it totally ruined my life (I'm not joking). I could barely function, had to give up my job, everything. And the rest. And I didn't have depression or your situation.
I would seriously advise against it. I'm restraining myself from giving all the reasons, but you said it - it is proper killing, and I know, as went through it. : (
So: next thing. Men, a lot of men, totally freak at a pregnancy. They go on and on for weeks, hoping for an easy exit (silent, simple abortion) They think it'll be far easier to pick up the pieces (i.e., you) than have the baby.
But if you just keep saying 'I love you and would do anything for you apart from that', they sort of get the message. When you actually just carry on with the pregnancy, they do get the message.
And when the baby is born ............ they generally love him or her. TBH I personally don't know of any guy who doesn't love their kid. Why shouldn't they? Just because they didn't give birth, doesn't mean to say it's not their kid.
So (again): I think the strongest likelihood, whatever he says now, is that he'll end up loving the baby just fine. However, he may be angry with you.
It's hard to tell, without knowing you both. I just think that abortion isn't really an option for you. So this now is about managing the pregnancy and him and your life, and being ready just to weather whatever storm you have to, to have, love and protect your new baby.
I hope it goes ok. This is just a transition period. Trust me, from what you've said you shouldn't for one second consider abortion. Forget it.
(I'm risking my neck by saying that, because on here you're mostly not allowed to be pro-pregnancy - there's this whole crazy 'a fetus is just a ball of cells and has no rights and is nothing' thing going on - absolute bollocks, I'm afraid, is what I think of that!) (The Emperor's new clothes with bells on)
Disclaimer: The opinion expressed here is only mine, and I realise that many MNetters will disagree and think I'm an idiot. That's ok : D