OP, this is far more common than you think/feel, though that wont make you feel any better today. Some things to hold on to.
A great disappointment in life is when we discover our parents arent perfect. She'll get over that... (or if she doesnt, then there is one spoiled narcissistic 'child' pulling your chain for the next 40 years). Secondly, whether she likes it or not, she will indeed one day be 30 and grown up. Its a further huge disappointment to realise we're utterly personally responsible. She'll get over that too. (Unless youre paying her car insurance, or subsidising rent/bills when shes 30, in which case you'll still be 'ruining her life'!
This is all part and parcel of growing up, and its the hard part. Go with the flow and re-focus your day to day activities elswhere. 200 years ago, she may have run off to be a cabin boy to be the grown up she desperately wants to be. 100 years ago, she may have joined up to fight the great war etc etc etc (I know I know shes a girl, but you get my point) None of those late teen plans include aspiring to be hovering round mums skirts, bringing her tea or picking her flowers. Its utterly normal, and Im just concerned that you are indeed reacting to some perceived guilt you may have from your perceived 'broken home' childhoods.
Youve done a great job. Youve raised great children. None are in care .....or prison. Great news that shes going to Uni in september, meeting others who indeed have come from far more chaotic backgrounds, is a great leveller.
My only point of advice Id strongly urge, is its for you to ensure the back door is left open. Youre the parent, and its for you to swallow the hurt and do the right thing. Sharing your upset or distress is for you to do with privately with your partner, not your children. Youre the adult in this relationship, not the one seeking comfort from the child.
Keep that back door open, drop a chatty email filled with irrelevant day to day chat once a week, and dont expect a chatty email in return. Your phone calls should be light day to day chatty stuff too, simply ending with a light, love you and miss you message. All previous insults, blame laying episodes should be utterly forgotten.
Shes 19, or rather 14 going on 30. As sure as eggs are eggs, she will grow up, and no doubt be a wonderful adult daughter as long as all her teenage failings are utterly forgotten and shes never reminded of them again. Only you can show her that that will be the case.