Sadly no method is 100% sure. Unless you get someone to finish you off.
Guy survived falling 84 stories.
Person survived a plane crash - TWICE
Many people have survived shooting themselves in the head.
I know someone who did not hang himself propely- he ended up paralyzed.
I personally tried to CO myself 3 times with charcoal. Followed every god damn step in the book. Never came close to killing myself. Maybe all I got is a fuvked up body.
The human will to survive is too strong.
My advise is plan very well. And have a back-up followup in the event your primary method fails.