Don't take the blame
If you loved someone at some point, ever, and you're a mildly compassionate human, you might be temped to deliver the softest blow possible via some variant of "it's not you, it's me". While you're probably shaking your head at the notion that you'd ever use such a lame line, chances are you've fallen back on some thinly-veiled version of that shtick before.
Pseudo-compassionate lines NOT to use include:
- I'm just not capable of love/serious commitments/monogamy (Subtext: WITH YOU).
- I'm not good enough for you. (Be ready for an impassioned speech from her side arguing why you ARE good enough, leading your right back to square one. Also, even if this is true, it's not the reason YOU are leaving HER.)
- I love you but I'm not in love with you. (This is the easiest way to confuse the hell out of someone.)
- I think we just need some time apart. (Like, forever? Don't string her along by calling a breakup a break, unless you want to have the same conversation in a month's time.)
The most important aspect of an effective breakup is clarity, which is exactly what the above lines lack.