Thank you for sharing this. I'm sorry you've been struggling for so long.
It's interesting though, back when I used to think about suicide, this method was the one I was drawn to. Probably because it's painless. I never got the point of trying it though. Sometimes I would make plans, not really planning to do them, like you say, idly. I would think of all the ways I could be found before it worked. All it would take was to be home alone with a car, but thankfully that never really happened, since I didn't have a car of my own.
Maybe my OCPD works in my favor on this front... I plan everything. And when you plan suicide, I think you usually eventually realize why it's not a good idea, and back out. For me, I can't stop thinking about my mom. My father died when I was 14 (they had been divorced for a decade but still loved each other), and when I think about how devastating that was, and it was just an accident... I just don't think I could do that to my mom, my sister. No matter how bad it's been, I guess I've never been delusional enough to think that my own mom wouldn't be broken by that. I could convince myself that everyone else wouldn't care...but it always came back to her.