I know there isn't much I can say that helps, but I've been in a similar dark place. I gave birth to twin boys that ended up having a random genetic mutation resulting in a disease called Tuberous Sclerosis. In their case, it is really severe, and we are dealing with multiple special needs in each of them. The disease, the medical complications, the twins, the expense, the stress, the darkness....it was just too much.
You'll make it through this.
You WILL. It's going to be hard, and you're going to go thorugh all the stages of grief and mourning, but on the other end you're going to find strength, hope, and love like you've never known. Stay close to your husband--this is IMPORTANT. Stay close, share your fears, and listen to his when you can. Lean on each other, and go to each other. You will need him, his love, and his support like no one else's over the next weeks, months, and years. This is the time to turn toward him, not away.
Special needs children often start life out rough. Trial by fire, almost literally. If you can make it through this NICU time, day by day, you'll be able to make it through the rest. One challenge after the next, and in the beginning they're big challenges grouped really close together. As you go on, the challenges will be a mix of small and big ones, and they'll be spaced out more. Much more. You will learn to cope, and special needs mothering will become as natural to you as the mothering you've always known.
Lean on the women here (but not so much that you're not leaning on your husband, too). They will help you through the dark times.
And don't ever beat yourself up for feeling things like "we can't take him back." My boys are 5, and I still have dark moments of wishing the worst....wishing I had an abortion, wishing they would die, wishing I could have my old life back, wishing I had never met my husband or started down this life's road, wishing I could find them new parents. These are the rare, deepdarkawful thoughts, and they're okay. They happen. They're pretty normal, from what I've seen.
They don't affect at all the other 99.9% of the time (now) that I feel love, patience, and thankfulness for them. So if in the beginning you have some dark thoughts, or don't want to be near him sometimes, let that shame/guilt/whatever roll off your back. It's okay. Those thoughts might come back to you time to time for years, but like everything else that's overwhelming right now, they'll lessen and you'll eventually see their place in it all.
Is there a grief counselor that works with the NICU? Someone you could talk to about having a baby with special needs? Sometimes they're wonderful people to talk to, sometimes they're hacks.
It's worth trying, on the off chance that you meet someone who gives you a word, or a thought, that helps you peek out of the darkness for a little while.
Wishing you and your family healing and strength.