I'm so sorry for your loss. As a victim, participant and now counselor in substances abuse, I deeply feel your pain. The death of a loved one from drug use is often more complicated than the common 5 stages of death most of us are familiar with. We're left to wonder things like, who gave him the drugs, how did he feel that he had to take drugs, what were his last moments like, what would life have looked like if not for opioids? And it's that much more difficult to talk because of the social stigma around the loss. It's one thing to say you've lost someone to cancer, another entirely to admit (maybe for the 1st time to others) that drugs killed the person you love. Others often get lost in the chasm between sympathy and awkwardness.
I know, because I think it's true of all of us, that your son would like to see you thriving and happy. He'd prefer his legacy to be one of hope, not one of pain. But you should know this: You did not cause your son to use drugs, you did not fail him in any way and you could not have prevented his addiction or death.
Regret is trying to change the past, and we can't do that. This is the time for pausing in place, for feeling and processing and screaming and crying and yelling and shaking your fists at God. And then it's time to get on with life. It'll be time to start the process of rebuilding a new and beautiful - and slightly more melancholy - life. Time. As cliche as it is, time will begin to heal over the rawness, but the process is slow and refining and stinging. I know the only thing that would make things right is to have your son back, but this simply isn't possible. Maybe the next best thing you can do is help create the person in you, the environment and the kind of world your son would be proud of. Good luck and be gentle with yourself.