Yes, I've felt this too. I think for me, it is when I really start thinking about a method and going through the anticipated pain/possible ways it could fail.
I think many of us have had suicidal ideation for a long time- so the thought of life ending feels more comfortable (welcoming) than frightening.
Still, the options available to many of us are limited and quite often violent. Think it makes sense we would naturally be afraid of pain/physical harm coming to us.
I feel like- when I start to feel fear, it's because I'm leaning more towards taking action and actually taking my life rather than just daydreaming it was over.
Some people seem to develop a calmness before they actually do it. I only hope this happens if I one day manage to do it. I just wish guaranteed painless methods were more readily available. It would still be pretty daunting but nothing like the fear we have now I expect.