Girlfriend's anxiety is crushing me

I have been with this girl for close to two years, we've known each other for close to six. I love her dearly, but I feel like I don't have a partner. We only moved in together this past September and I'm at the end of my rope.

Her anxiety completely consumes her and she shuts down completely if she's away from me. When her stress level reached a certain point she will completely break down. She has not harmed herself physically yet, but has done so in the past. During that time, she will say horrible, horrible things to me. Usually about how I'm no good and that I'm not helping her enough. How stressed she is and generally just freaking out. I always ask her. "How can I help?" And I try to calm her down as best as I can, but nothing seems to work the same and I feel like I have to walk on eggshells around her to avoid provoking anything that will cause her the tiniest bit of stress. I'm actually close to loosing my job due to the time I've had to invest in getting her out the door and off to school. (College)

I guess what this post is asking is, how can I help her further? How can I help her tame her terrible anxiety? She always apologizes and feels horrible afterwards. I need advice and all of it will be gratefully accepted.

UPDATE: The talk went pretty much as expected...not well. There were fireworks, screaming, doors slamming. I'm not ashamed to admit that I cried during it. We agreed to work together and keep trying before she went home for the weekend...now she's saying she's stuck on how I can't be responsible for her mental health...which was the whole point of the fight. I said I will help her, but I can't manage her mental illness for her.

This has been my first long term relationship and I'm feeling a little jaded. Thank you all honestly for your advice. I appreciate it more than I can describe.