You’re doing the right thing, ending that long-term relationship that’s going nowhere. Try to ignore how deeply you love him, and know that you’ll painfully grieve and mourn the loss. You’ll be starting your life over again, in many ways, and it’ll be a long time before you adjust.
Don’t fall into the rebound arms of that guy you’ve known, as a casual friend, for a while. He’ll seem wonderful at first, you’re vulnerable, and a heady new distraction will fill the void and ease your pain. But, he’s a bad person with personality disorders. When you see him at that party, just say “hi” and move along.