Why am I here?

Or is it more like what am I doing?
Im in a new relationship, but Ive been thinking, and I think that i don't want to be in it...
I'm aware that I need to analyse myself and my motives for this, e.g. not try to make this new into a 'new maddy' or a substitute of any kind, and I haven't been...but I realise that I still miss her. Whenever I'm with this new girl, my mind's not in the same room. It's off thinking about Mads again. Missing her and still loving her.
You'd have thought that love, unreciprocated for over 5 months, would have died by now, having no nourishment from the person you so love (i mean, her love for me died after two weeks, apparently), but it still clings into the memory of her. Wishing. Wanting. Missing. Loving, unconditionally. It tares me apart because u know she doesn't want to come back.
Again and again, I wish I could go back in time, tell myself to beware of this coming emotional void and to fight it with all you've got, or to smack my formerly in-void self to snap out of it as he doesn't realise how much he'll loose and that it's ok to keep her because they love each other so. But I can't.
i can only try that here, now. And she doesn't talk to me. I'd imagine she's trying to make a new life with that bf of hers and trying (if not already succeeded) to put me behind her.

I have to so something. I have to end this mistake. And I wish She'd forgive me for the ones I've made. I'm never going to make them again. Ever. I just know. I prommisse.