I don't think you're supposed to forget about him, but for both of your sakes maybe you're supposed to try understand you can not help him. Only he can do it.
Sorry I'm blunt here. I was were you are for the logest time ever, but at some point I've realized he was only a part of the problem, the rest of it was me. Me, who coulnd't get it into my thick head he's a separate person, he can do with his life whatever he pleases regardless of how poor his choices might be.
Once I started working on myself, trying to figure out why I'm the way I am, why am I so desperately in need of someone make me feel complete, things started to get better for me.
So I realized I've spent too much time crying why am I banging my head against the wall, only to realize there are other options there, maybe I don't like them, but they are there: I could keep my head banging and cry in vain or I can more away from the wall, as one thing is for sure: The wall is not moving because I'm banging on it.
I know it's hard. You're not supposed to stop caring for him, but sure as hell you're supposed to start caring for youself.
Sorry if this sounds harsh. I wish you best.