Letting Go

I can let go of a job I never got, of an insult someone said, of a stupid decision that I've made. Of a lousy grade that I got, of practically any mistake that I've made...but the one thing that I can't seem to get over...the one person that I can't seem to let go of...is the thing that hurts me the most. When I had him, he didn't really make me happy, and to this day...all he does is depress me. All he does is make me feel worthless and he brings out the worst in me. But when I'm talking to him, or when I'm with him, I feel complete. I feel...quiescent. And even though he can make me as doleful as can be, he is the only thing that gives me true blissfulness. Music makes me happy, yes, but usually if the song reminds me of him.
Even before I met him, I knew there was something missing. Someone missing. Someone who understood me more than anyone. Someone who SAW me when no one else did. Someone who knew that I was amazing while I thought I was nothing. He used to be that guy. He used to be there when there was no one else. But now he's left me. And I thought, surely I would eventually get over it. That I would get used to life without him. But it isn't so. And I look at everyone else around me, and I see them letting go and moving on while I'm stuck. I feel so alone. Even the guy himself has moved on. And at the end of the day, this whole thing is...pointless. Loving him will get me nowhere. Missing him will only suffocate my soul. Thinking of him will only take over my mind. And all this really does is pull me away from what is really important: God.
So is this the Devil himself trying to keep me away from the light? Or is this truly my heart reaching out for something that isn't there?
I've tried to replace him with God. So that I will transfer my love from him to God. That I will transform my thoughts of him to thoughts of God. But I only end up failing. There are moments that I succeed, but every day when I wake up it's as if those moments never happened, and I have to start all over again. It's a constant struggle, a constant war. Battling the darkness to keep the light alive...