I'm afraid I have no advice, but I think you that you're handling this perfectly.
I'm getting on in years now, mid-60's, but I still remember the devastation and shame I felt at letting my father down as a teen, and as a young adult. Like your daughter, I was extremely bright. I was always top of the class, loved learning and had my heart set on Oxbridge. Then I entered 6th form and everything went to hell in a hand cart. I didn't finish 6th form, didn't even get through the first year. Since then I've tried (and failed to finish) more degree, and other higher education courses (both bricks and mortar, and distance learning), than I care to admit. Not because I can't do the work, (I always get excellent marks), but because I cannot seem to cope with unstructured learning. I've only ever completed one higher education course, and that was only a year long course to start with, but my then husband also enrolled and I think having that anchor helped enormously. I've felt shame and disappointment in myself all my life because I couldn't do this one 'simple' thing. Then, in my early 60's I was diagnosed as autistic (and was told I probably also have inattentive ADHD, as well), and everything suddenly made perfect sense. I still feel disappointment, but it's not inwardly directed anymore. I'm just sad that I was born in an age where autism wasn't as well known as it is now, and at a time when 'autism in girls' was virtually unheard of! Had I known then, what I know now, I may well have been able to find the right support to enable me to cope with unstructured learning.
Reading your post made me so sad because your daughter sounds a lot like me (except she managed to get a bit further, so kudos to her for that!); She did really well in school (structured learning), she didn't do as well in 6th form (semi-structured learning), and couldn't cope at all with uni (unstructured learning). If, as you suspect, your daughter is also autistic, even acknowledging that, without a formal diagnosis (NHS adult autism and ADHD assessments are swamped at the moment, and many trusts have suspended their wait lists entirely, so for many it's an 'expensive, private route' option only, right now), but even without a formal diagnosis, the knowledge alone would be a massive help. Even in areas where you think she's doing ok or excelling, she may be struggling more than you think (we females are experts in masking), As an adult, help is scarce, even non-existent, in most areas, but FE and HE establishments have wonderful SEN support, and a formal diagnosis is not usually needed to access that support. Even if she decides not to go back into education, the knowledge could massively help her in any career she decides to pursue. I second the book recommendation made by a previous poster ('Aspergirls' , by Rudy Simone), and would add 'Odd Girl Out' by Laura James.
You're doing a brilliant job, and despite some posters telling you otherwise, you're being the exact sort of mother she needs; supportive and non-confrontational. Whatever she decides to do going forward, whichever route she goes down, having someone supportive at her back will be an enormous help. Despite my own craving for one, degrees aren't the be all and end all, and they don't guarantee a good career. As an example, my son hated school and decided he didn't want to go onto further education and so left school at 16, with quite poor exam results. I worried for him, even though he was never out of work, initially he seemed to have no focus or drive. Fast forward to now, and he's a very successful, management level, programmer/analyst, at a top international company, earning a 6 figure salary (and he's probably also autistic...), then there's my friends daughter who flew through uni and emerged the other side with a 1.1, and has since drifted from one dead end job to another, and currently working at Burger King at nearly 40,
Whatever your daughter decides she wants to do, I'm sure, with your loving support, she'll do fine. Good luck to you and your daughter!