While I don't have an answer myself, I knew several people who wanted to CTB and ended up not following through with it.
My father tried to ctb in front of me with a gun but was physically restrained. So I think instead of directly carrying the act out, he turned to risky behaviors instead, as others have mentioned. Drugs, alcohol, keeping company with dangerous people, were all things my father partook in.
I can remember being left at my elementary school many times in the afternoons because my father must have been binging the night before and forgot to pick me up. In the end, he was killed by one of the sleazy people he was friends with, his life cut short at a young age.
Yet even as a child, I knew that my father was hurting very badly and wished for his life to end. He was trapped in an unwinnable situation. My mother, who pretty much gave me up as a baby, had a strong desire to CTB as well and when she wrote a letter to me explaining why she didn't raise her kids, she told me that it never gets better. I think she is very religious and that is why she chose not to end her life, even if it is miserable.
A close friend of mine's mother suffered from years of trauma, involuntarily psych commitments, and extreme mental anguish. Like many of us here she had tried everything and wasn't seeing any improvements. She took it out on her children and her family until the bitter end, when she became terminally ill and suffered greatly in an attempt to find some sort of cure. She died never knowing peace and contentment, and I find her story chilling to the core.
I am scared to end up in a situation like that where it will be physically impossible to end my life if I become incapacitated. You always hear the motivation porn stories in the media, those who have horrible ailments and were in a dark place, but now they're so glad they chose life. They never show the stories of those who spent everyday suffering and didn't get a modicum of happiness out of the whole ordeal, and I find that reality quite horrifying.