I know you feel guilty about the past. About the things you wish you would have done differently with him. I feel the same way and my kids aren't even grown up yet.
The thing is, you can't change it. The past is over and done and there is nothing you can do today to make up for it or make it right. I'm not saying it's good or bad, right or wrong, it's just over. The worst thing you can do is continue the behaviors that led to the current situation. It appears that's exactly what is happening.
You have no power over the past. You can only change the present.
L