kiper, you seriously want to break up with her? has the fire run out / the water run dry, so to speak?
guys would normally say the line (to borrow from odell) "you deserve someone better", but to tell you frankly, this line is so used...and i'm sure your gf is smart enough to see through that, right? sometimes it can be annoying how guys seem to underestimate the intelligence of the female species...
if you really want to end it nicely and not add more damage to the break-up, you can do it gradually...small hints, inconsistencies, etc...make her feel that it's you who's wrong (caveat: since you want to break up wit her, you have to carry the brunt of being the bad guy, even if you're not) - i'm not saying that this happens all the time, but it's better than just telling her flat out that you've fallen out of love - if you do this, she'll plague you with the question "why?" and she might not accept / belive it right away, she may even try to persuade you into thinking that you're temporarily deluded and that you'll get back together eventually. girls are a mean specie, kiper. hehehe...
but of course, the best judge would have to be you coz you're the one who knows how your woman will handle it. good luck in breaking up (sounds weird? kinda paradoxical, huh?)
Just tell her you need some space. That you two need to grow up in separate ways. Sometimes "change" is good. Just look her in the eyes and tell her the truth. You just have to be adults to discuss this matter maturely. Good Luck.