Is this sentence grammatically correct? "I knew you must meant to love"?

No. It is not grammatically correct. Quite what it should be is hard to say as it really doesn't make sense. It might benefit from a verb: "I knew you must BE meant to love"?

The problem with it is there is no apparent verb. If the verb is 'to mean' the placement of must is wrong. Must have meant would work.

I knew you were meant to love.

The placing of some words make that sentence grammatically incorrect. This should simplify the sentence "I knew you were meant to love".

It would help to know what you were trying to say.
I know you must be meant to love (but you're denying yourself that ability)
I knew you must have meant to be loving (but you failed)
I know you must mean to be loving (but you're not)
I knew you were trying to be loving (but you weren't)

Include 'have' between must and meant then it should be OK

Depending on what you are trying to convey I would say "I knew you were meant to love me."

No. You could say, "I knew you must be meant to love".