If you're having second thoughts, don't force yourself to do anything. Wait until you are sure. Death will always be there for you later. That is something I tell myself when I'm distraught. Sorry, it's probably strange that I find the thought comforting. But perhaps it will reassure you too, or whoever else reads this.
If my options were SN and N, I would choose N in an instant. Maybe I would take both: first the N; then immediately prior to passing out, I would chug a SN shot. No such thing as overkill (bad pun, okay) when it comes to suicide.
I can't say if it is painless or not as I've never tried it. And as far as I know, everyone who has tried it has died. But if it is anything like overdose of other substances - bam, lights out. The issue would be if you are found before you die.