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LW, no matter you choose, it sounds like your marriage has already hit that land mine. Get the abortion, and your husband will resent you. Have the kid, and you will resent your husband. Either way, unless you're both committed to do some serious work, your marriage doesn't sound like it will be happy or long-lasting. And even then, success is not guaranteed.
Look, sit down with a friend or a therapist or a spiritual leader and talk over what you want, what the likely outcomes are, and which of them seems best for you and your twins (though make sure that your friend doesn't have a knee-jerk response on abortion). We're strangers here, talking apart the sentences in your letter. How can we possibly tell you which road to take?
I do think, though, that there are a few questions you should grapple with, preferably sooner rather than later. Why did you let yourself get talked out of having your tubes tied and/or having a hysterectomy? What do you mean that you and your husband were still "discussing" having a child, and if you are sure that you don't want another one, why was this even being discussed after 8 years? Were you using birth control, and if not, why not?
I have my own theories on the matter, but I don't know you. Good luck, LW, 'cuz God knows you'll need it.