Firstly, just keep in mind that "premature" or "early" covers a whole range of gestations - a 24wker is completely different to a 32wk baby. Don't be frightened by the possibility that a 32wk old baby would be as poorly (and need as much intervention) as a really early baby.
I would also recommend the diary - even if its just basic information, comments that the Dr made. As the others have said, there will be days when you have a bit of a set back (which feels like going back to Square 1 even though in reality its just a temporary blip) and being able to look back at the very early days to realise just how far they've come can really help.
Be there as much as you can - I can see you have other children so its difficult, but you'll find that there is a doctors round at a set time each day and then a weekly consultants round. If you're there when those are happening, you can hear the nurses feedback a progress report and get a better idea of what is going on. I also got to understand the language, the medical terms and on one occasion spoke up about something that I thought they'd missed.
As others have said, don't think you're unusual if you feel cheated / hard done to / jealous of pregnant women / new mums and also be prepared for mums / babies that you've got to know in SCBU going homw before you. Its a real emotional rollercoaster which you can feel like you're on by yourself. I was also pretty horrible to my dearest and dearest - I think at the time I was just so desperate for time with them / to hold them that I didn't like other people coming to the hospital or asking to hold them because if they only had 20 minutes out of the incubators each day or whatever, I wanted all 20 minutes - not grandma or great aunt whoever. I think people understood.
Also, remember that you will get to take your baby home. They told me when my twins were born more than 12 weeks early that it would probably be around their due date when I'd get to take them home - I couldn't comprehend that we'd be in limbo so to speak for 3 months - it just seemed like a lifetime. It seems strange but you quickly get into a routine - the time will go quite quickly with the whole expressing / nappy changing / feeding / trying to eat / sleep etc. In fact, we came home after 8.5wks in SCBU. Would also agree that in general, the SCBU staff are amazing, you will get to know them very well and they will usually bend over backwards to let you be the parent, you've just got to make sure you make it known that you want to do as much as you can.
My twins are almost 6 now and it seems a distant / hazy memory. It seems like the fact that they were premature will shape their lives (and yours) but it doesn't.
Good luck
Mandy