Why are you ashmed? What did you do wrong? Did you force the drugs down him? I feel for you. Never second guess your feelings. If you let him go you are going to hate or dislike him that is grieving.
Someone told me you have let the things you love go to set them free. If it was meant to be it will come back.
I have been where you are at for the last 15 years. When my XAH left me for the drugs and the dealer that was suppling them to him. I always wondered why, still do at times. But I do dewell on the answers anymore.
But you never are going to know why or the If I only. You need to tell the people that are close to you. When you are ready to help yourself you will let go. But it will never be easy to let go of someone you love.
Take it on day at a time.