A 'break' is a funny concept, because I don't think anyone really knows what it is. People often get confused as to what their boundaries are and what the general etiquette is when you go on a 'break'. The whole point of a 'break' is to have some time out, to give yourself some space so you can think things through. Therefore, it can be very hard to put a time limit on it. From my experience/from what I've seen, 'breaks' tend to be unsuccessful and just prolong the inevitable. Unfortunately, the human race cling onto anything that gives us hope, and that's exactly what a 'break' does.
Anyway, sorry, I've digressed. In regards to your situation, I think it's very unfair that your girlfriend wants to go on a break because of her mental illness. Reason I say this, is that I'm sure she'll have more episodes of bad anxiety throughout her life and she can't tell you that she wants to go on a break every time! Perhaps, she's using her mental illness as a disguise to simply cover up the fact that she doesn't like you in that way anymore. (Just a thought).
At the end of the day, there is very little you can do as by the sounds of it she's already made up her mind.
I think the best thing you can do is just go with it and perhaps when you get back off your holiday and still want to be with her, drop her a text and just say that you've been thinking about her and that you miss her and see where it goes from there.