I have a similar ish (6 year) gap between mine.
My first thought is - explain less, listen more. Ask her how she feels, and show her you accept those feelings (even though it can be quite difficult for you to hear) - yes, sometimes she wishes the baby wasn’t there? Yes, in lots of ways it was really nice being an only child and having you to herself? Yes, the baby is boring or annoying and sometimes she has to wait and she feels not as much loved by you... acknowledge it, don’t make her feel she has to bottle that stuff up. My rule has been, feeling badly towards the baby is fine and normal, just don’t act badly towards the baby (or anyone else for that matter).
I read a really good analogy that took some of the things we tell our eldest children about why we had another baby, and turned them into statements about a spouse/partner - that was really helpful. Nobody would be thrilled if their DH brought home a new, more charming, adored-by-everyone wife and said things like “I need you to wait while I do blah blah with her first”, “it’s not that you weren’t enough for me, it’s that I love having you so much I wanted another too”, “don’t be silly, of course you don’t hate her, she’s part of our family now” etc.
It’s a fine line to tread sometimes because I would never say to DC1 “sometimes I wish DC2 wasn’t here either” (that wouldn’t be true or helpful), but where possible I do acknowledge that i really loved our years together before DC2 came along too. Or I’ll vocalise out loud, seemingly to the baby but for DC1’s benefit, “shhh, I know, I know, you’re very small and you just need to be held right now; but this is DC1’s bedtime and all that crying is spoiling it; DC1 is being so patient with you; he’s my baby too and he still needs me just as much as you do; I’m really looking forward to when you are a bit bigger and you go to bed earlier so I can have bedtime with just DC1” blah blah.
It’s the hardest thing when they both need you at once. I always try to externalise my thought process, because otherwise DC1 notices when he is the one to wait (but doesn’t always recognise that I know this too and I hate it too), but doesn’t spot the times when DC2 is also having to wait.
They’re now 7 and 1 and absolutely adore each other. For us, the hardest time was during my pregnancy - he hated the idea of a baby coming along and spoiling his happy setup. He still has moments of ambivalence and I’m super proud of how well he can articulate them to me whilst still being very tolerant and patient with her. The toddler also now gets a bit jealous eg if the big one is on my lap, and again I gently and acceptingly point that out to them both.
Hope something in this ramble is helpful... I remember finding ‘Siblings without rivalry’ a really good read, but i haven’t really looked at it since she was tiny so my memory of what it actually says is hazy!
IMO it really helps as the baby gets older and more and more interested in the sibling - really flatters their ego, and I totally bigged that up whenever I spotted it (“oh, she doesn’t laugh like that for anyone else! You’re definitely her favourite person blah blah”). I also have to stop myself from interfering sometimes - always watching and ready to leap across the room if I need to, but actually it turns out the baby isn’t so fragile (I get that that may not feel so true, and indeed may not be so true, given your youngest had a difficult start).
I tried to make sure every screech of “careful!” or request to wait a minute while I dealt with the baby was outnumbered by lots of positive interaction with the eldest, finding fun things we could do together while I only had one arm free, basically really digging deep even though I was tired as hell, to make him not feel he was being short-changed. The baby’s needs were huge but also very simple - cuddled, fed, clean, warm. The big one was the complicated one, so as long as the baby was cuddled/fed/clean/warm the big one took priority. I actually saw this as an investment in their future relationship, because in many ways I think balancing their needs became harder, rather than easier, as the little one grew bigger (sorry).
Wishing you all the best.