I like how you prepare to desensitize yourself by practicing and I think that is the best you can do to prepare your mind and body for the attempt. I think part of the reason for fearing death may tie into the fear of non-existence and just not having any conscious (part of the survival instinct). Also in regards to the fear that you mentioned, it is the survival instinct part of your brain subconsciously trying to deter you from following through. However, I believe with enough willpower, desensitization (through practice and mental drills of the attempt like you mentioned), it is possible to override that stupid SI and go through with it.