Oh MJ, I'm so very very sorry
No matter what, he knows that you are taking care of him, and we all know that too. You cannot take on the responsibility of not curing cancer, that's just too huge. Even with all the decisions that you have made, you can't second think those either, because you got him this far, and there is never any crystal ball.
I understand the losing it part, but do know that this is not on you. This is on the cancer. F*ck cancer.
Please let us help you through this.