Ahh all the emotions

I am so stupidly obsessed it's not funny. 
I've slept with him a few times. He introduced me to many sexual pleasures. Including anal, which mind you I absolutely LOVE. 
He is my (dead) best friend from high schools older brother. We kind of found comfort in each other after my friend, his brother, passed away. 
The problem is that he has an on again off again relationship with this woman who brings out the worst in him it is really upsetting. He knows she is no good for him and that she is just using him for money and what not but cannot seem to get rid of her. 
I'm falling in love with the guy. He know how I feel about him. And is happy to keep fooling around like we do thank god, but tells me he just isn't a good enough guy for me long term. Which isn't true. He is the sweetest guy I've met. I just wish he could see it and not listen to this other girl who tells him all the time he is a worthless guy with no potential. 
So I guess what I'm saying is, how do you win someone's affection that already has a thing for you physically and somewhat emotionally but not mentally!
We have all the same/similar interests and yet we cannot be together. 
I don't know if I'm asking for advice or I'm just upset tonight and don't want to deal with it.