i don't know what you mean. do you mean you fall in love with her when you see her first? in that case, "너에게 첫눈에 반했어" you can say

ok first of all, the english is not correct it should be " i have had a crush on you for the longest time or just " i have a crush on you. This is is how you say it in KOrean 나 고백하고 싶어요 I have a confession to make, 난 너에게 반했어요 I have a crush on you or 나 너 좋아해요 I like you,

Your phrase doesn't make much sense in English. Could you please rewrite it?