The best way is simply to separate the two.
You don't need a "good argument" for why you'd like to break up. Relationships are an at-will agreement in all 50 States plus Guam(*) – you can be fired at any time, for any reason. So, don't bother to explain it in any great detail.
Of course, it's a little weird to simply walk out on someone, especially after several months or years of a relationship. But just like firing an employee, it's best to keep things minimal and neutral – to use such clichéd terms like "it just isn't working out", or "we're moving in different directions".
It's not dishonest, it's simply avoiding a fight that benefits no one, and potentially extends your involvement in a weird limbo-state in which you're convinced you're broken up, and the other party is still hanging on for dear life, or perhaps trying to scrape off a pound of flesh on the way out the door.
Honestly, the question itself suggests a desire for confrontation, to "get the story out"… But to what end? If one needs to vent frustration with a failed relationship, that's better done with a support network of friends who will remain with you, take your side, or even (if they really like you) even challenge some of your assumptions.
It's also incredibly zero-sum… A breakup needn't be the result of a failed relationship. You can end on a draw, or a mutual "win". And if you can do that, why wouldn't you? It just makes you look even better to potential future candidates.
When things run their natural course, and it's time for them to come to an end, go out classy.
(* I'm joking about Guam. I have no idea what their policy is.)