Who should say I love you first the boy or the girl?

What if the other doesn't love as much?! What should they say?! I'm with this girl we are together but not official for about 3-4 months now and I'm gonna ask her out but we have grown a lot together. She's my first love, she's my first girlfriend, I'm 21 btw she's 20 and I know eventually she's gonna leave me, it's reality but also you nvr know what could happen. And I'm afraid to lose her, but back to the question usually who says I love you first