is it okay to say..

He or she will probably understand the last three words.

Candid . . . "i'm going to be candid with you"--and you don't want the word "only" in that sentence. You might say "i'm just going to be candid with you", but you don't really need that, either.

How about: I'm going to be honest with you. I love you.

" im only going to use candor with you, i love you."

'candor' is a noun. As has already been noted, the word you want in this case, with 'be' is the adjective form 'candid'.

You could also use the adverb form as in,

" im only going to speak candidly with you, i love you."

It's 'candour' in non-US English speaking countries. Not to be confused with candida, which is an embarrassing condition. Or Candiru, which is a Brazilian prison, or 500 kV power station, or something.

Sounds a bit like pillow talk to me. What's wrong with, 'Brace yourself?'

she will miss the rest and only hear the last 3 words...odd but still sweet