✔ 最佳答案
Let's look at the sentence:
... against the man I once believed ___ the love of my life
And focus only on the adjectival clause and yank out the rest of the words that play no significant role in the meaning of the sentence:
... against the man I once believed __ the love of my life
Now, it is very clear that you adjectival clause already has a verb, "believed." Thus, you cannot put "was" in ___ since that would get you two main verbs in one phrase.
Regarding "as," it is easier for me to explain if I flip your sentence back from an adjectival phrase to a simple sentence.
... the man (whom) I believed as the love of my life
... I believed the man as the love of my life.
This does not sound right either. What you want to say is probably:
... I believed the man to be the love of my life.
Then, if you flip the sentence back to an adjectival phrase, you get:
... the man (whom) I believed to be the love of my life
And the phrase is now grammatically correct.