Your problem goes way deeper than a child failing in school! You are living in a toxic environment where no one will succeed or be happy!
I speak from experience. I had a situation much like yours. Only I was stupid!!! I had excuses not to leave..I can't afford it, etc etc, so I endured it for 25 years. Probably to survive, I jumped off the fruitless merry-go-round and stopped focusing so much on him, because it was so miserable.
I took control of my own life, and stopped trying to be responsible for his kids failures or for him. I basically gave all that burden to him, and emotionally backed out. I still gave the kids care and love, but I stopped agonizing about their situation in life. I would always tell him to handle it and remind him that I would not. I forced him to share in the housework, and grocery shopping with threats to leave. He stayed horrible to live with, but I was busy "escaping", so It was ok.
That single act of "opting out" kept me from going crazy. I told myself I was going to have my own money, and I took more control of my life by getting a job and squirreling away money. That whole time I let him support me. I never shared my money with him. Then there was an an incident that was the straw that broke the camels back. It propelled me to get out for good.
By that time, I grown enough to become detached from him (and his drama) and had been for years. So after the initial exhilaration wore off, there I was a 62 year old woman, wondering where the hell my life went.
I needed to start the healing process, and get on with life. Through counseling and lots of prayer, three years later, I am reborn and happier than I have ever been.
I still regret that I essentially threw away my life. I don't know why I waited so many years to leave. Maybe with counseling, I would have gotten out 15 years earlier. But my future is bright now so I look ahead, not backward.
Don't be like me, and be a senior citizen when you get your life back!! Unless you take action, this is your life for the rest of your life!. Get some counseling, get strong and get the hell out of there!