It is good that she's being honest with you, and warning you now. I actually think that this sort of honesty is a sign of a good advisor. For example, when I popped over to Cardiff to meet with a potential supervisor, he was a great guy and we got along well but he kept emphasising, "this project is going to involve a LOT of programming. You've got to be comfortable with coding- and fast- and I've had a PhD student drop out because they couldn't pick it up quick enough so I need you to be capable enough to do this." Didn't put me off wanting to apply with him but it was a bit terrifying. However, if your supervisor (like this dude from Cardiff) is honest from the get-go, then you know that when you're working on the PhD, they'll be honest about what they think then too- and that's the way by which people end up with great theses! I know it's easy to take these things personally (as I mentioned a few pages ago in fact, haha) but she's only saying this because it's both to your and her benefit that you know what the PhD involves and are prepared for it. When things do go wrong (which, as both of my supervisors insist on telling me, definitely will. Over and over again
) perhaps try and figure out what your best coping mechanism for those times is? I know things aren't as straightforward as that but it's good that you're going into a situation where you know what could go wrong and so can prepare yourself for those times, at least a little.
One last thing I'll say that my post-doc supervisor once said to me and that really stuck with me: although it's reasonable to care about what your supervisor thinks of you, ultimately this PhD is for
you not for them and at every step it's about what you're getting out of it- what skills you're learning, etc. Because she can't fire you, she's stuck with you now, she said yes to your application and so now it's all about you! And I'm sure you'll do fantastically well and impress her anyway.
I'll stop rambling now, haha.
EDIT: You probably knew all the above anyway- I'm just procrastinating from work now haha. And Piggsil managed to say what I wanted to say but in a much less long-winded fashion.
(Looking at my thesis, long-windedness is an issue...)