J.L.
I know somewhat how you feel. I had a 2 year old and 10 month old twins when I found out I was pregnant AGAIN! (my first 3 were IVF so you can imagine my surprise). Talk about feeling overwhelmed!!! I also had a husband that worked outside all day and did absolutely nothing but sat on the couch and napped on and off all evening. Asking him for help was just another source of stress for me. I soon stopped asking.
I remember at one of first appointments with the 4th as the Dr. was searching for the heartbeat, he couldn't readily find it and was lying there secretly thinking, "good, maybe there is no heartbeat, I can't handle another baby". But he found it soon after. I also remember being embarrassed to go out in public with all my babies and my big ol' belly. People still ask to this day if they are all mine. (I had all 4 babies in 3 years).
My point is, no matter overwhelmed you feel right now, you will love this child, you will be able to take care of him/her, and you will be a more amazing mom than ever. Just hang in there, don't be afraid to tell your Dr. how you feel. He is the most qualified person to help you at this stage.
My son is now 6 and I love him so much, we have an amazing bond, even closer than the older three. We just get each other.
Prayers to you and your family.