This is to you. Yes, you who is confused and emotionally unstable. To the person who don't know what's he's doing, who is trying to heal himself, who's trying to live in the moment by ignoring the fact that this moment you are living in right now can have a serious effect on someone. These moments can turn out to be monuments on someone's heart and soul, they might haunt them when you'll leave.
Look! You know you can never love her the way she deserves to be loved, you know you can never forget that one person from your past, you can never give her what she wants, isn't it? So why let someone live on the false hope, why gift someone your heart when you are going to take it back later, why do you need to show her that you love her when in real you don't? Why tell me exactly why?
Okay! You don't want to hurt her by telling the truth but how long can a lie survive, how long can a relationship/bond live on a fake foundation, sooner or later it'll break. And your lies might her more than your truth. Truth hurts just one time whereas the lie, it hurts and it lets the other person resort to hate and no one loves to be hated. Your truth might set her free, so give her freedom by telling the truth, don't keep her shackled in the name of fake love she'll get suffocated and frustrated.
So, this is the right time, let her know just now. This is to you to set her free from your lies, set her free from the love that'll not serve her for long. Set her free and let her be and then she might come back again, not with any hope but with peace in her mind.