I've no lack of knowledge, intellectually, about why people do what they do. This however boggles my mind.

My girlfriend of the last year, now ex of about an hour, seemed to have a total mental breakdown and then simply reboot her whole night and smile and seem as though nothing changed. At first it was clearly an act but after a short while she seemed fine, more so then one would expect as she has had in the past and does sometimes suffer from sever bouts of depression.

My question, as I am worried for her health, is what route should I take from here?

Hmm, your ex-girlfriend is acting as if nothing changed as in the break up right? And this occurred after a breakdown that she had? If so, I've seen people react this way because they are in denial. They think that just pretending that the break up didn't happen will make you go with the flow.
You asked how to go about this...as in how to tell her that you broke up with her? If what I am assuming is correct... I would suggest that you have a mutual friend, maybe, know about this break up and slowly ease your ex-girlfriend into realizing that you won't go with the flow and keep continuing this relationship just because she acts like nothing happened.
It could be really harsh for her, especially since you said she has gone through depression. However, I think if you intend to break up with her...then you should keep doing so instead of getting back with her. It would be more hurtful for her to continue a relationship this way.

....hopefully I understood correctly. If not, please do correct me ^0^

Yeah, something along those lines. Our roommate is a mutual friend who is aware of the situation and such. I'll have to wait and see what happens until tomorrow though. Still debating on whether or not I want to sleep on the couch or chance going up and actually sleeping in my bed.

I, myself, have been single for quite some time (because, apparently, according to friends, I am too darn picky), so I am hesitant to offer up my advice. However, I have been in my fair share of breakups.

People cope in a whole range of ways and perhaps the about face was a defense mechanism or, rather, coping mechanism to lessen the blow. The hard part is your not knowing how this will blow over once she realizes that she has to face the hard fact the relationship is truly over. I do know, however, a mistake some people make is trying to ease the other party into the breakup slowly by still spending time or acting "lovey-dovey". While it is often done with good intentions, it ends up hindering the person by lengthening the pain, anger and denial stage and delaying the important stage of renewal. As cruel as it may seem, there is definite wisdom in a clean break. Once you have both established separate lives and moved forward, you both can choose to resume a friendship, if so inclined.

With that, I agree with SSA's advice. However, it should be a friend that you feel she would be comfortable confiding in or she may feel ganged up upon, like, say, a mutual girl-friend that she trusts. Also, it should be a neutral, understanding friend that won't bad mouth involved parties. No matter who or why fault is involved, a friend should listen and guide towards moving forward, not dwell. This friend can then be there during the lows so that you can initiate distance. Of course, I realize this may be difficult if you both live together, however, if possible, distance yourselves right away by either of you staying at a friend's place or whatever for a couple of days. This should be a time of trust, no one touches the others things, changes locks, whatever, with the intention of setting a time to talk again maturely after this "cool off" period to see where you both stand after the time apart. Sometimes that final meetup is best done at a private place outside of the residence. This tends to counter the tendency to overreact and maintain real discussion. Don't make any further promises: I'll call you, I'll still see you often. While it is okay to reiterate your feelings for each other - because you obviously cared for each other once - making promises plants a seed of hope which may make it harder to let go.

*nods* I think Emiko pointed out the most important part. Having a friend that the girl can feel comforted by...and begin to accept the reality of the situation.

Being a part isn't a problem, we have a third bedroom that we've been using for storage. Worst case I'll just clean it up and move her in there. As for a mutual friend, our roommate is the best option.

The only other option is her moving back in with her mother. If that's what it is, well that's what it is.

Well, thanks for the advice and I'll leave an update if anyone wants one. Who knows.

Turns out she was in denial. I put it in straight terms about 30 min ago. Now to just wait for her to calm down and check on here every now and then.

It is a good thing you were straight up with her. I hope the situation turns out well.

She's taking it better then I thought she would. A lot of crying but we're going to work out what's going to happen over the next couple days.

Its never easy, even if the decision is a mutual one. I'm happy, for the both of you, that the worst seems to be over and that moving forward as individuals - and hopefully somewhat friends - can now occur. My best wishes to both of you!

<3 L