Battle of the Break-Ups Part 1

Every relationship will fail... except for one. :)
Keeping this in mind, many relationships eventually come to a point when they need to be ended. As my mother always says, "A relationship should end when it becomes more negative than positive." If your relationship is at this point there is a proper way to handle it. Just be sure that this person is not The One before taking the drastic measure of The Break Up.
I will actually be using my own mistake for this demonstration:

Towards the end of a relationship, you may become resentful of your significant other. In my case, Scooter became a woman (figuratively) and I was not willing to wear the pants in the relationship (not that I could squeeze into his skinny jeans, anyway). I personally am a bit old-fashioned in the idea, but I believe men should not wear more make-up than I do or spend more time getting ready in the morning than I do over the course of an entire week. Being fed up, I finally took my position as the man (after much dry heaving, but it had to be done) and I did what many men have done before. After a normal date, we said goodbye. Only he didn't realize that I had no intention of ever seeing him again. I celebrated a small victory of regaining my freedom. A month later, after meeting Marshall, Scooter texted me begging for an explanation and asking me to come back. I stood my ground and told him no. Looking back now, that was the only right thing that I did.

What you (I) should have said:

Everyone deserves an explanation. Even if the explanation is, "You are just annoying the hell out of me and I just can't see you anymore." I don't suggest this approach, but it's better than nothing. You MUST also follow this by de-friending her on Facebook and trying to stay FAR away from your ex for as long as possible so that she can mend and move on.
If you cannot be that blunt, there is a way to handle this situation with minimum damage done (well, as minimum as possible when feelings are involved). I am personally a fan of the truth. In fact, one of my past boyfriends sat me down and said, "I'm sorry. You are a wonderful person, but you are not The One for me. I don't want to hurt your feelings but by continuing dating I could be keeping you from meeting The One for you." I was hurt and I cried for a while, but after moving I finally realized that he was right. I've actually messaged him to thank him for being a gentleman and helping me find a good possible candidate for Mr. Right (here's hoping things work with Marshall and he doesn't find this creepy XD) Even if Marshall's not the end, some wonderful man out there is and you get the gist. It all works out for the best. :)
The most important step here that I want to stress is that you MUST leave your ex alone so that she can move on and find happiness and so that you can too. This way when you have the "past relationships" talk with the girl of your dreams (see Ghosts of girlfriends past) it will be smooth sailing. :) Best of luck!