The one who is sleeping beside you now. She who makes sure you're well covered. She who ensures you have enough to eat. She who holds your hand. She who'll catch you when you fall. The one who keeps all your dirty secrets and speak up for you all the time, who will jump to your defence regardless of the situation. She who fits nicely in your arms. The one who will accept all your childishness and playfulness.
You are letting go of she for a "her" who you know will never match up.