I suppose the problem is- we simply don't know how painful it will be to die. Even if you read 1000 people's descriptions/ witness reports, it couldn't guarantee you the same experience.
Worse though- it may hurt to try to suicide and we fail and end up with more health complications.
I suppose I try to tell myself that- if my time comes to suicide, I will convince myself I am sparing myself the pain of growing old and ill and dying naturally but- both are unknowns ultimately. It may have been my fate to die peacefully and randomly in my sleep. Or, be hit by a bus or something. It's all a gamble at the end of the day.
I think it makes sense that we fear the unknown. And that we fear potentially worse pain because of a failed attempt. I wish it were nothing to worry ourselves over but ultimately, we can't know.