I know that feeling of sadness. It's overwhealming at times.
I often look back and play the "what if" game with myself wondering what I could have done different. The game never ends, and I keep on spinning. I'm tired of the guilt and sadness. I am slowly letting go of it. It is helping, and my son is actually doing a bit better ( fingers crossed).
Maybe he is going better because I'm letting go more? Not sure. But I am feeling better.
I wish for you some peace. You deserve it.