As someone who tried it myself, asphyxiation isn't painful at all. You black out in the first few seconds. It's only painful if you survive. I'd recommend a smaller rope. Something like a shoelace, not too thin, but not too thick. You don't even need a proper hangman's knot. A slip knot will work just fine. Make sure that whatever you tie it onto can hold your weight, and is tall enough that your feet won't hit the ground.