I love you with all my heart.
我全心全意爱你.
I love you for you.
我真心爱你!（简单吧!当你想表示爱的是对方的人,而不是其他如钱财,权势等,就可以这样说）
My love for you is as deep as the sea.
对你的爱,似海深.
I'll love you as long as I live.
爱你一辈子（爱你千万年）.
If the sun were to rise in the west, I'd never change my mind to love you forever.
即使太阳从西边出来,我对你的爱也不会改变.（顺便温习一下虚拟语气吧）
You're the only man/woman I could be happy with.
（这句话是不是太夸张?）