It's difficult for sure, and different for everyone. I almost went all the way with a handgun CTB earlier this month, but couldn't actually make myself do it. After looking back and thinking about it, I felt like it wasn't SI that got in the way, but just imagining how "gory" it would be (even if I wouldn't see it), and how painful it would be if it went wrong. I picked it because it seemed like a simple method and easy for me to obtain, but the reality of the attempt showed me things I hadn't considered. I also realized that in that moment, if I could have pressed a button for an instant, painless CTB...I think I absolutely would have done it. So it's not always totally about SI.
But now I'm going for SN, because it feels a lot easier/peaceful to take a drink than to pull the trigger of a gun aimed at myself. I can't be sure how I will respond when the time comes, but I feel at peace about it.
It also helps that not a moment has gone by from my aborted handgun attempt when I wish I had been able to CTB that night. Those feelings have given me motivation and peace about my next attempt.