Mrshectic - what a horrible position to find yourself in
big unMNetty hugs, you sound like you could do with them
I don't have any experience of anything like this but just didn't want to leave this post sitting here unanswered.
There was a similar thread to this a few days ago. There the overwhelming response was that you are probably a lot more likely to regret going through with an abortion than having the child.
What do you think your dh would say if you turned around and said, you know what, I think you are wrong. we will get through this like we originally said we would and I'm sorry but I can't go through with an abortion. Are you worried he would leave you alone with the 3 children, or become more depressed or get into massive debt and/or something else?
It seems to be from your OP that he was the one that instigated the appointments for this, that you hadn't thought of it until he suddenly changed his mind and decided that you needed to get rid of the baby. Just because he changed his mind, it doesn't give him the right to automatically have that be the accepted vote of what is happening. You also have the right to have your views and feelings - and as it is your body then you do have more rights because he can't force you physically to have the abortion (he can't physically drag you to the clinic and tell the doctor to perform the procedure regardless of what you think about it).
Bizarrely, it's probably easier for him to force you to have the abortion mentally by manipulating you (however he does this - threatening to leave or getting even more depressed and so on) into making you think that that is the only option you have open to you, thus sublimating your feelings and desires in order to appease him and keep him happy and the rest of the family ticking over.
Have you told him yet that you have cancelled the appointment for weds? If so what was his reaction - or what do you think (or fear?) it will be? Relief or anger? And is he going to be on the phone organising another appointment for you or leaving it up to you?
I'm not surprised you feel you're going insane - this would be hard enough to deal with on its own, let alone with lots of pg hormones swilling around, being so busy at work and stress at home from dh's long term depression and unemployment.
Sorry that there are so many questions but sometimes it's only asking lots of questions that will help you figure out what you want, rather than falling in with what everybody else wants because that's what easiest at that horribly stressful busy moment in time.