You will never know that how much I love you语法很完整
但把that去掉表述也正确,而且听起来更地道一些.所以建议使用
you will never know how much i love you.
语气更强一些可用
You can never know how much I love you.
英文差也没关系,这两个句子足以让你表达了,任你选了.