just don't love you anymore. (诚恳但是太伤人心)
我不再爱你了.

It's really not working. (有理有据,还算婉转)
我们的感情真的行不通.

I've met someone else. (简直是找死,人品不行啊)
我已另结新欢了.

We've grown apart. (哟,这个不错.理性!)
我们都各自成长了.

The magic's gone from our relationship. (太"文"了,好像演戏)
我们之间的爱情魔力已经不见了.

I think we should be just friends. (够直接,意思表达得很清楚)
我想我们应该当朋友就好了.

It's not you, it's me. (好!自己承担后果.有大将之风)
不是你的问题,而是我自己的问题.

I really don't wanna be tied down. (确实是理由,万不得已的时候再说吧)
我真的不想被绑住.

You're really too good for me. (这个不错,婉转,一般都用这个)
我真的配不上你.

You don't really love me anyway. (哈,够直接了)
反正你也不是真的很爱我.