Translation:If only you knew how important you are to me.
This sentence also makes sense if the first word is לא, which is what I heard...
It is implied and does not necesserily needs to be written .. In Hebrew you can say אילו/לו רק ידעת כמה אתה חשוב לי or אילו/לו ידעת כמה אתה חשוב לי and it would mean the same (perhaps the first a bit more desperate) just like in English - 'if only you knew how much you mean to me/how important you are to me' or 'if you knew how much you mean to me/how important you are to me'
So if we go back to the lessons notes, this sentence would probably not be used in everyday speech, and we would probably say: אם רק ידעת. Correct?