"(like me and most of us if we were confronted by her hellish circumstance) wanted to die."
So that must mean that she also wanted to die.
First problem I have with your analysis, is that you have no way of knowing what she was going through.
Second problem I see, is that you are making the presumption that if you wouldn't want to live the way that you think she was living, she also did not want to live like that. Who are you to decide?
She may well have determined that, while it was not the best way to live, that she still wanted to live anyway.
Two different issues. For myself, I am the one to decide. That's why we all need living wills to try to diminish the opportunity for well-intentioned, but misled people to get involved and try to frustrate our wishes. As to Terri (or any person other than ourselves), we have a two-step analysis. We have to have a societal mechanism (we call it 'law') to determine what they wanted for themselves. That is precisely what was done in Terri's case. Those are the legal processes. We can keep tweaking the process, but the determination always has to be their wishes.
Then. we have to move to the misguided "moral" arguments about implementing her wishes, our own desires again become relevant because Christ told us, "In everything, therefore, treat people the same way you want them to treat you, for this is the Law and the Prophets." So, in any moral analysis, our own desires for how we would be treated is critical.
She may well have determined that, while it was not the best way to live, that she still wanted to live anyway.
Sure, all things known to human experience are possible, but there is absolutely no evidence that she changed her mind before the cardiac arrest. Moreover, this was not a woman who simply lost the power of speech and kept other cognitive abilities by which she might have changed her mind after the cardiac arrest. Her brain was destroyed, so she could no longer think and she was incapable of changing her mind (once the cardiac arrest occurred). Very sad situation. But to protect the sanctity of her earlier wishes, we have to have system that makes a good determination of her last known wishes.