Drastic changes are signs of insecurity, especially when they follow a breakup. They're often a diversion tactic, used to lead attention away from the more serious issues at hand. Not only that, but they are temporary "fixes" much like substance abuse, and only serve to prolong the inevitable.

It takes a lot of time for one to grow their hair long. Therefore, their hair will be with them through each and every experience along the way...joys, sorrows, frustrations, conflictions...so there is, in a sense, a lot of "energy" stored in the hair. Its condition will also reflect said experiences and therefore tell the story of your life.
    When people cut their long hair short, they often feel relief, as in being set free from their past because they are parting with the "energy" from all of those years before, which they feel is stored in their hair.

I, however, would never cut my hair drastically for anything other than pure necessity. Several years ago, I had to do just that after a terrible colouring fiasco. I went from waist length to shoulder length and finally to 6" all over just to remove every trace of unnatural colour.    I didn't want to part with the length, but it was a necessary sacrifice.
    From that point on, I vowed to grow back every inch that I had lost, and take much better care of it in the process.

I would never cut or alter my hair in any way on a whim, because I know that I would instantly regret it. Salons are my hair's natural enemy anyway. Nor would I ever cut it after a breakup. Please! No guy on earth is worth that.    Life has thrown some pretty horrible things my way. Lots of them, actually. But I choose to deal with them directly as they come along and not take it out on my hair. The only way to take the pain away is to confront it, not run from it or distract yourself from it.

As for Britney, who cares? Seriously.