"Should I tell her how I really am or continue to try and be supportive and cry it out later? I actually feel like she's been abusing me, she's gets so rude, shouts, tells me I'm stupid and crazy, everything I do is wrong etc."
Something I say often on here:
If you do what you've always done, you'll get what you always got.
So, if being supportive gets you shouted at and insulted - why would you have any expectation that continuing to try and be supportive would achieve anything else other than being shouted at and insulted?
So if you want any other outcome, you're going to have to take a different approach. That doesn't mean blaming her for your mental health, but I think it should involve a drawing back from being the all-supportive mother. It's time for tough love. Still supportive, but support becomes more emotional support and less financial support, if you see what I mean. They're still your child, you still love them, but you're loosening the apron strings to let them strike out on their own towards independence.
"she's a teenager, she's vulnerable."
She's 19 - no longer a child. Take a deep breath and allow her her adulthood. Why do you consider her vulnerable? Is this something specific to her, or your anxiety talking?
"This afternoon I've scheduled a 'chat about the future'."
This chat should start by asking her what she wants to do with the rest of her life. Make no suggestions. If she offers none herself, by all means ask more specific questions such as 'Well how do you plan to support yourself financially?' but don't suggest anything. This is what adulthood is about, making your own choices and your own mistakes.
Make it as positive you can. 'You're 19 now, It's not like you'll still be living with me when you're 40 (make it a nice high ridiculous age so she doesn't leap to the idea that you're throwing her out and start screaming at you), you'll want your own home and life, where do you see yourself in ten years time? How do you see yourself reaching that point?'
Keep asking questions, giving her the benefit of the doubt that she's been asking herself these already.
And yes, there should be sanctions for rudeness and insults. That is just plain tantrumming, and is the action of a toddler, not a 19-year old adult. She wants to be treated like an adult, it's time she behaved like one. She shouts insults at you, you walk away and remove one of the benefits of being in the household, like laundry or cooking - because why would you want to do anything like that for someone who's being nasty to you?
But above all else - accept she's now an adult, tell her you want to move towards treating each other as adult to adult, not child to mother/punchbag.