Hopefully you're all home together now, but in case other MNers read this in future...
DS2 spent 3 weeks in hospital when he was born (35 weeks, but only 3lbs 5ozs due to rubbish blood supply). Ds1 was 3.6. It was grim.
Can only echo what everyone else says - try not to torture yourself, I did and it does no good whatsoever. Take as much help as you're offered, let standards drop as much as you can.
Try to spend a bit of time with your eldest every day, and explain if they're old enough - DS1 was devoted to his baby brother from the beginning, and we told him as best we could why we were worried. Accept you might get a bit of regression (DS1 started wetting himself again after being dry for months) - that's not your parenting, it's their reaction to stress. DC notice more than you think.
Cry when you need to. Have a glass of wine in the evenings and try to enjoy the single lonely positive of SCBU - time to rest and recuperate without getting up to a newborn at all hours.
DO NOT turn yourself into a demented, obsessive loon WRT trying to get bf started (if this sounds like the voice of bitter experience, it should...). But the Analytical Armadillo blogger is AMAZING if you need some advice.
When in hospital, get out and have a walk in natural daylight every couple of hours. Hospitals are weird and draining places.
Never, ever let yourself feel that your LO is less your baby because they're not with you. Our nurses were very firm from the beginning that he was ours, and we were responsible for his care. He recognised my heartbeat from the first day I could hold him and used to calm down instantly.
Be prepared to take a while to recover from the experience - but remember the baby won't ever know a thing about it.
For what it's worth, DS2 is now 4 - healthy, charming, stubborn, fearsomely articulate and you would never ever know it had happened. He enjoys the stories of how he had to stay in the hospital in a special box, but a story is all it is.
for everyone going through this.