She […] claims that she has often tried to convince herself that she is exaggerating everything
Same. Both before and after my adult ASD diagnosis, TBH. Less so now, though, almost a decade on. I don't think about it much these days, and in my head I take a fairly neutral view about what I do and don't "have", neurodevelopmentally — though I acknowledge I meet the criteria and that specialists agree (and I can still ruminate on it when I'm mentally unwell). If I need to disclose, I phrase it as "I have an ASD/ADHD diagnosis", which both avoids taking a position on it in terms of correctness or personal identity, and lends it more credibility in a world of self-DX.
I vacillated a fair bit about the whole being-autistic thing, even after I got a solid NHS diagnostic report from a team headed by a highly specialist neurodevelopmental psychiatrist, who had access to 15 years' worth of my psychiatric notes, plus oodles of questionnaires for me and my parents, and hours of interview. And I still don't totally buy into my (later) ADHD diagnosis. (Happy to have the drugs, though…)
I mean, surely school wasn't that bad, even if I hoped each schoolnight that I wouldn't ever wake up from as far back as I remember, had serious behaviour problems that seemed inexplicable to staff, got kicked out at 10, left another school for the mental hospital at 15, and dropped out of college twice, and anyway I had no excuse for that behaviour and the social misery was entirely my own fault…
…and clearly I'm just finding excuses for myself both in the present and in the past, for what I've put my loved ones through over the years, and am making a big deal out of normal little things when everyone has quirks and difficulties, and anyway isn't it possible I've trained myself into this somehow or am unconsciously putting it on, or I've got learned helplessness, and doesn't it seem more likely I'm actually just wilfully crap at stuff rather than disabled…
…and anyway maybe I met the ASD criteria because the things I'm shit at just coincidentally line up with the things autistic people often struggle with, but the difference is autistic people can't help it, and besides which, isn't it kind of circular anyway, like "why am I shit at this stuff — it's because I'm autistic — how do we know I'm autistic — because I'm shit at this stuff", and also it could be that they only diagnosed me because I knew too much about autism and behaved the "right" way and gave the "right" answers subconsciously-on-purpose, and there are so many things that I can do just fine that real autistic people struggle with, and obviously if I put more effort in I could do the things that I prefer to believe my autism makes difficult…
…so maybe I'm just lazy, attention-seeking, gullible, manipulative, unlikeable, selfish, tactless, lack resilience…
Or alternatively:
I know I have this diagnosis but I don't want to be autistic, that would mean there was no hope I could ever be different, this means I have a faulty, substandard brain that doesn't work like it should [even though I don't think like that about other autistic people, obviously!], so maybe I should try being not autistic-like, and instead of using autism as a pathetic excuse to be crap, realise that I'm not actually autistic and get on with it, not lumber myself with a lifelong condition, and I know I've never yet managed it but I must've not been trying all those previous times…
And on and on and on. I'm an overthinker. Maybe it's because I'm autistic, or maybe it's a personal flaw that I could get over it I only put in the effort 
Maybe your DD's thought process is similar, maybe not, but what you describe her saying — that self-doubt about your own subconscious motivations — is something I find very familiar, and I don't think it's that uncommon a thought to have, at least among later-diagnosed people.