I want to say to my girlfriend that she is everything for me. And my question is if I can say her this thing by the sentence "You are my whole".
I made a little googling about but I didn't find a clear answer.
The more idiomatic expressions that you hear used are "You make me whole" and "You complete me".
I had to think a bit about why that sounds funny to me. While whole is indeed both an adjective and a noun, so there isn't anything grammatically wrong with the sentence, the question "your whole what?" comes to mind. This is the reason for Subjunctive's comment. While we see what you are driving at, we just don't say this. She is your whole something, such as world or life.
"You complete me" would be very proper, but "You are my whole", based on the tendency of most to take things in a sexual nature might lead to you getting slapped...
"You are my whole" is not an English idiom, nor is it a complete sentence. Here are some similar poetic statements:
There is a song titled
You are my everything.
There is a popular country song whose refrain includes the lines:
There goes my life.
My future, my everything.
And another popular song's refrain includes the lines:
you are the one
That I've been searching for my whole life through
In American culture, if a young man says something like this to his girlfriend, he is taking a big risk. Many women want a man who has a life of his own, and is not dependent on a particular woman for his happiness. They appreciate it if they can make him happy -- but they might think less of him if he is not confident that he "can survive" "with or without" her.
In writing it sounds weird (because if she is "whole" it means that she's better off without you, so "my whole" makes no sense), and phonetically it's completely rude and vulgar.
I suggest you look for some other expression. There are a few good ones in other answers.