I think you really, really need to try to be strong here, turn your whole mind set around and be ultra positive about secondary school, coloured pens, fabulous pencil case and whatever else she needs. I think walking on egg shells and avoiding the topic because it's difficult is probably the worst way to handle it, because you're making it a bigger and scarier thing by it being 'the thing that no one can talk about'. The more it's talked about the better I think, just start in tiny steps.
Carry on the ultra positive even if she's not warming up to it - there's every chance she'll slowly come round. Be excited on her behalf, tell her the things you loved about secondary school, tell her about your favourite subject, tell her about what the crazy kids got up to and the funny things your teachers did, talk about what she's enjoyed at primary school and how much more of it she'll get at secondary school. Teach her how to tie a tie if she needs to learn, talk to her about the lay out of the school, look at the school website and find out any useful information from there that might help prepare her - obviously not all in one go so she's totally overwhelmed though.
Has she had visits to the school beforehand? DS had extra settling in visits before starting for anyone with SEN or who was very anxious, it helped so much. If she has remind her of those and talk about anything she did there that she enjoyed and how well she did. DS fell in love with the library not long after he started and the librarian is amazing - it's been his sanctuary for the last 7 years, he will miss it so much when he leaves. It would probably be great for her to have a safe space where she can decompress if she needs to so worth thinking about.
Has she got any primary school friends that she's moving up with that could come over to make her feel better or anyone she could go in on the first day with? Will she have someone from her primary school in her tutor group with her does she know? DS has always been quite happily a loner which worried me hugely at first but I had to accept he was ok - and now he's just started to catch up emotionally with his peers and is seeing that he can be sociable - just that certain ways of socialising suit him better than others. It just depends on your daughters personality.
Could you plan something special for the end of her first day - not anything big or busy but her favourite cake in front her favourite film when she gets home or something. Something to look forward to and decompress with at the end of the day.
Use the things she loves to make it easier - she loves art then get her loads of art stuff and talk about how great the art department will be for example. If she loves something random see if you can find a pencil case, rubbers etc with it on. Just try to make it all fun and positive.
I would also at some point talk through some of the problems she might face in a positive light and ways she could handle them - for example what would she do if she couldn't find her classroom? (Reception are really helpful in ds's school for help with finding your classroom). Don't panic! is always a good reminder and tell her that teachers will expect students to get lost and be late sometimes.
If you can give her ways to handle some of the problems she might face then she will feel much more prepared should they come up.
You could ask her what some of her worries are and think of ways to deal with them together. Think about what she might say beforehand though so if she says 'I'm worried no one will like me' you don't burst into tears! If she says that then remind her of the friends she had at primary school (if she did) or tell her everyone else will be worried about that too, it's perfectly normal and good ways to meet new friends are to chat to the person sitting next to you in class and get to know them, or join some clubs and chat to people there about what you're doing, or she could take something to do at breaktime if she doesn't want to be sociable or can't find a friend such as read a book or watch something on her phone,
Good luck! Hopefully the anticipation will have been much, much worse for you both than he actual event! If there are difficulties then don;t hesitate to contact the SENCO, tutor teacher etc and talk about what could be put in place to help.