Feel very sad for you that you’re in this situation OP, it isn’t easy at all.
Just to be open, I am normally against abortion unless there are valid risks to the mother’s life and/or medical issues with the baby that would mean a seriously lesser quality of life.
However, you already have two children to think of and support, you’re being logical about you capabilities and fears for your mental and physical health. Being a mother is really hard work and you’re realistic about what you can offer.
That said, your husband is against this, and I fear that this could tear such a hole between you that your marriage will end. And then what stress will that put on you and your children?
There is no winning situation here in terms of negative circumstances.
I think you need to have more conversations with your husband to see if you can work this out between you.
Either you go ahead with the abortion as planned and he supports you and you move on. Or, if there is no financial impediment, you decide to keep the baby, and you do a few things to make that easier - he takes paternity to look after baby/no or limited breast feeding and you get more help.