Health anxiety after losing a loved one + guilt he's still alive

My dad was incredibly healthy. He would only eat junk food a few times a year, and in small portions. He exercised everyday and rode a bicycle everywhere. He had hobbies and friends.

Suddenly he went into cardiac arrest. His heart condition misdiagnosed as indigestion / anxiety and an angiogram revealed he would need a quadruple bypass. But it was too late.

After the paramedics revived him and when he woke from his coma a few days later, we realised he had a severe brain injury. It's been almost 4 years and since that day he can't talk, sit upright, eat, or drink.

I don't know anyone who's gone through this. He was not particularly old - he was not retired yet.

He's still alive but I feel like I've lost him.

And because he has such a severe heart condition with no risk factors, it's believed to be genetic.

I've had symptoms such as severe chest pain episodes and fainting (even when I'm sitting down) and a resting heart rate of 95-100bpm. I've had a few cardiologist consultations and every doctor seems to think I don't have a heart condition but I have POTS (high heart rate and fainting) and a digestive issue causing the chest pain.

It's hard to believe doctors when they thought my dad was fine and now everyday I feel guilty he's alive and suffering.

(Please no advice relating to euthanasia and assisted suicide -it's illegal here)