Interesting question.
My last serious attempt was partial hanging under non-ideal conditions. I felt mentally calm and collected in the hours leading up. I had rehearsed during the previous nighttime as I was unguarded, I waited to be alone that morning. I had my usual cup of tea, then sat and reflected on my life and my family. I reflected upon the seriousness of my decision, but assured myself they would understand, and then I proceeded to set up the apparatus.
The most nervous I felt was in the immediate moment before I allowed the noose to tighten, I was apprehensive and unsure of what to expect. As it turned out, the actual experience was rather horrific before I fell unconscious. But again, that was under non-ideal circumstances and I was really attempting to escape an extreme situation, so my experiences may not really be representative.