I'd say act deliberately. You being up front and honest with her will sting her now, but she will eventually respect and appreciate how you treated her.

Edit: In response to your edit, I have some more insight: I used to be the guy who would break up with a girl and get back three days later because I got lonely, or would convince myself she needed me. I would do this 2 or 3 times to a girl, and eventually she just resented me. I resented me. So, now that I have learned from my mistakes, here are some do's and don'ts in breaking up in the manliest way possible:

Do

• Be resolute in your decision; no one wants a lack of confidence, even in difficult, emotional situations. Stick to your guns, especially when she asks you questions about why, why, why: "I'm not happy in this relationship." should suffice as your go to response. No need to go into ANY details, even if it is her fault. She may snore, laugh weird, talk with her hands, whatever your reasons are, they are irrrelevent to her. Why? Because she is no longer in your life, and you owe each other nothing.

• Hug her, but only if she initiates; you probably both need the hug and it could help solidify closure.

• End it at her place; it keeps her dignity in tact.

• Cut off all contact after - delete/block facebook, delete numbers, photos, and remove anything that reminds you of her; there's an exception to this, but only if you are both emotionally mature enough to handle being friends afterwards.

• Surround yourself with friends; it will make your transition back to the single life much, much easier.

Dont

• Relapse; you made the decision to quit the drug known as her, so stick to it.

Basically all the opposites of the "do's". I have faith you'll be ok.

One quick thing: this is something I do to help me get over a chick. I imagine her getting fucked, hard and fast by another guy, or even multiple guys. She cums multiple times and is enjoying herself. She is happy. I generally want my exes to be happy in life, and I hate the heavy jealousy I can sometimes feel, so I allow the worst thing that could ever permiate my mind to occur on purpose. I get it over with on my own terms. I think it so thoroughly that I ultimately end up in a better place because I experienced her being happy without me, and that makes me content.