"A personality disorder is not something that can be diagnosed in one appointment or even two."
^ This ^
My daughter has a diagnosis of NPD, with traits of EUPD, bipolar2 and possible ADHD (they weren't 100% certain of this diagnosis, apparently). She was 19 when she was diagnosed (she's now 24) - but that only came about because of her behaviour, which I've spoken about in my posts here, before. It took maybe 6 months of her seeing a psychologist for a diagnosis - and she only ended up having a MH assessment because of her violence towards and lies concerning me/her (younger) brother. Essentially, the police pushed for her to be assessed.
My daughter lies, rewrites history, gaslights, believes herself better than everyone, has no actual friends (and never has), manipulates, and remains abusive. We suspect past sexual abuse at the hands of either her paternal grandfather or paternal uncle - she'll hint at it (even to the point of blaming me for not safeguarding her contact with them), then claim not to remember anything ever happening to her, so cannot categorically pinpoint where her illness began. Her last boyfriend literally ran away to the opposite end of the country to get away from her, having had to call the police because he was afraid of her - then refused to press charges against her, which I understand. Her life, as a young woman, is pretty lonely.
I recognised symptoms of bipolar in her from the time she was 9 or 10 years old (I'm bipolar, as is my father, as was his mother), and I repeatedly asked (begged, actually, towards the end) our GP for help - but nothing was done. I was told I was overreacting, due to my own MH illness. Not even the footprint left embedded into my thigh, in the form of a bruise, convinced the GP that my daughter needed help. She was 15 or 16, then. I knew that she was ill - but hadn't actually heard of EUPD back then, whilst narcissism? Was simply a word used to describe vain people...
Whilst my daughter's diagnosis did little to change her behaviours, it reassured me that she wasn't malicious - she was (is) ill. She literally can't help a lot of her behaviours, and has had therapy to help with coping mechanisms. She still lives at home (a condition for which is that she has to be medicated) despite some posters on here advising me to kick her out (when she had nowhere else to go). I do worry that when she moves out, however, she'll stop taking her medication - because she doesn't believe that she needs to be on anything, - and will hurt someone, who will press charges against her. But I also know that won't be my fault/responsibility. I've done everything I can to access help for her, and whilst she's under my roof, she's as stable as she's ever likely to be. Hormones do make her more verbally abusive, however, so once a month there is a barrage of accusations flung my way.
OP, if your sons are anything like mine is, be prepared for them to choose to have very little (if anything) to do with your daughter, as they age. My son is loathe to be in the same room as my daughter for very long, because of what she did to us - and can't even confront her about it, (a) for fear of setting her off, and (b) because he knows that she's literally written it out of her mind. He's angry with her - and I respect that, just as he respects my choice to understand that she is ill. That her fluctuations of moods are not her choice, but something that she cannot control at all.
I am, however, more than aware that when my daughter claims to be suicidal it's to either get her own way, or to manipulate others into staying a part of her life. Self-harming is also something that she's claimed to do - only to insist that she's never done "such things" to herself, when challenged about it (she also has no scarring indicative of having done so).
If your daughter is mentally unstable, OP, then you need to talk to your GP and push for a diagnosis. Doesn't matter if she's over 18, you can still talk to her GP about her - they just can't talk to you about her, if that makes sense? But please be aware that if she has a PD - it's not something that will make her life all sunshine and roses. It's not a diagnosis that any parent would willingly want their child burdoned by.
PM me if you like. And good luck.