You don't know when it happened, you noticed it over time, maybe it was something you saw or heard, but u realized it, you don't love them anymore. You did, at some point, but
not now, probably
never again. You hate giving this person the
false hope of your love, you know they deserve better, you come to terms and accept the fact that you don't love this person anymore.
I'm sorry, I don't love you anymore.